Experience! Gold! Thieves! - Kinder Surprise Houserules/Mods.

Your kind of party, eh?
Russ Nicholson for Citadel of Chaos.

Quicker Advancement.

• If you're playing with only three classes (OD&D style), shift down the XP tables so MUs advance as Fighters, Fighters as Clerics and Clerics as Thieves (or unchanged), or:
• Use 3e/4e/5e XP tables, and subtract that amount from corresponding older school class XP requirements. So, a BECMI Cleric needs 500 (3e) or 1200 (5e), a Fighter needs 1000 (3e) or 1700 (5e) to hit 2nd level. 

(This is one way of reducing the economy-busting amounts of gold that are an issue for some, and a way of speeding up advancement for the time-poor while keeping the staggered class advancement model)

Bonus Extra: if the table agrees, every time you roll a save, you get XP equal to the amount you missed or made the save by x10. Roll exactly your save value for 100 XP.

It's more book-keeping, but rewards general adventuring behaviour/risk-taking as you get something whether you fail or succeed. As your characters rise in level, it will make much less impact and you can drop it once it gets too cheese-paring.

Less Cash in Circulation.

Related to the lower XP requirement idea above and the fairly common trope that adventurers/heroes start off each story broke, hoping to make one big score and then end up cheated and broke again.

Characters do not keep ANY treasure that goes towards XP. 

It is out of your hands by the time the next adventure comes around, through debt, carousing, alms-giving, theft, gifts, living expenses (at the appropriate level of extravagance), cheats, gambling etc. You can even call it training costs. Narrate or hand wave this as your table likes.

Any cash (or choice items) you want to keep (in hand, in the bank, buried under a particular tree, put towards buying a castle, whatever) or spend on adventuring assistance/supplies does not count towards XP.

For extra misery, allow included living expenses to exceed the amount of XP you gain, so you end up in debt.

Are healing and curse removal costs inclusive or extras? 

More Thief Mods.

Because we just can't help it! 

Under either of the following (and I don't think they can work together), keep Hear Noise as a d6 skill (if that's in the system) and Backstab is unchanged.

Skills as saves: allocate your Thief skills to your saving throws (combine as appropriate), and test them on a d20 (or convert to %). 

You can swap a pair of saves/skills every time you gain a level.

You can apply Ability Score adjustments as agreed/appropriate.

Skills as hit rolls: each skill is an attack roll vs. unarmoured AC (or convert to %).

Your skill advances as a Fighter's attack progression, and leather/chain/plate can be used to represent grades of abnormal difficulty or increasingly stressful conditions.

In a mod of 2e AD&D weapon specialisation, the Thief can spend max. one of their NWP slots to specialise in a single skill at 1st level and get a +1 bonus. They can use a single additional slot gained at higher levels to specialise in another skill, or take their bonus to max. +2. This might also work with the 'skills as saves' method too.

Bonus Extras:

• Thieves open doors, locks, chests, disarm traps etc on an Open Doors roll, but use INT and/or DEX (depending on characterisation, and Dragonwarriors would suggest an average) to generate a STR bonus equivalent.
• Keep percentile skills, but you get to add your Prime Requisite % bonus to the base value.
• Thief skills % start at equivalent of 3rd level and scale from there.

Commentary.

Quick crude mods for a basic D&D adjacent game. Not playtested but someone's probably already tried all/most/some of these already in the last 40+ years.

Realise I could have held them back to pay this Joesky Tax I've heard about, but they've been hanging around my drafts and notes for a while now so I'm putting them out rather than letting them fester.

XP for saves is a rip from T&T.

Old School Thief Variant - Adventurer, Ranger, Rogue.

There comes a time in all our lives when we have to 'fix' the Old School Thief.

Considering they're part of the core text of D&D, they needed a lot of modding.

Description: Thief, adventurer, ranger, rogue, ruffian, swashbuckler, scoundrel, hero.

Whatever they call you, your traditional ancestors are still Cugel and the Grey Mouser. But roll high STR/ CON and you're also Conan or Fafhrd without having to multiclass or otherwise bend/break the rules.

You might want this kind of Thief to specify if they're more inclined to the rural, the urban or the dungeon environment, and tweak their abilities accordingly.

XP, Saving Throws, & Attacks: All as Thief.

Hit Dice: Depending on the base system, you start with 2d4 or 2d6 hit points, but gain only 1d4 or 1d6 per level as normal thereafter.

Prime Requisite: Dexterity.

Armour: Any, but as Thief only when you want to do Thief stuff.

Weapons: Any, but you prefer the lighter/smaller hand weapons for special ability reasons.

Alignment Restrictions: None, unless that's important for in your setting/system.

Special Abilities.

Thief Skills: Everything a Thief can normally do, but agree which of either Hear Noise probabilities or boosted percentiles you use before play begins.

If you have a CHA, INT or WIS bonus (highest applies), you can start casting from scrolls that many levels earlier but chances of failure do not change until your levels catch up.

Snipe: You can Backstab using a missile weapon if this is not already allowed.

Dual Wield: When fighting with two weapons, you get +1 to Hit or AC. This also applies to unarmed strikes. You cannot Dual Wield while performing a Backstab.

Choose which bonus to use at the beginning of each combat round, before you roll any dice. 

If you hit, use the higher rolled damage or choose which weapon inflicted the wound (for magic and poisoned weapons, for example)

If you have 18 STR, you can wield a max. d8 and d6 weapon simultaneously; STR 15, 16 or 17, a max. d8 and d4 or d6 and d6; all other STR scores, the max. is d6 and d4.

(This presumes you don't already have Dual Wielding going on anyway)

Cleave: If Fighters get extra attacks vs. <1HD opponents, so do you but only when Dual Wielding.

Surprise: If you are alone or with no more than three other Thieves in leather or no armour, your first surprise attacks count as Backstabs.

Other People's Stuff - Survivor Levelling & Hit Dice vs. Damage.

Two things that I encountered this week that I liked/gave me food for thought.

Survivor Levelling.

On Twitter, Claytonian (of the Kill It With Fire! blog) shared the latest version of their A Carcosan Hack this week, and I was taken with this little house-rule gem from the advancement subsystem: 

'To be honest, we just adopted a system where if a PC dies the other PCs level up'

I really like this idea, either as addition/ alternative to XP counting and/or milestone advancement. It makes a narrative sense I find pleasing, as well as encouraging both risky and cautious play.

Claytonian says that the obvious PvP exploit hasn't been a problem in play.

Has anyone else used something similar, or are there published games that took this route?

HD vs. Damage.

Doing a little searching for whale and whaling related gaming material brought me to this post from 2016 (the blog has not been updated since 2018). 

To quote the author: 

for those new to Dungeons and Dragons terminology, “Hit Dice” – often abbreviated HD – are an abstract representation of toughness and fighting skill. when a Player or Monster is injured in Dungeons and Dragons, the attacker rolls an appropriate number of dice for damage – tougher fighters and graver injuries call for more dice or dice of larger size (a 12 sided dice, instead of a 6 sided dice, for example). The defender then rolls their Hit Dice, one at a time, until either they have totaled enough to survive the attack, or they have exhausted their pool of Hit Dice and are unable to fight on. [sic & my emphasis]

This is not a method I have encountered before, though I have been thinking about something not unlike it, on and off - see 'Logical Progression from Abstract hp?' (at the end of the post).

It's one of those things that I could have missed completely during rpg hiatus: was this ever a thing in core D&D (any edition)? Is it a house-rule or a mis-interpretation? Does it come from the assumptions of a different gateway system that D&D?

Sort of Thought of Basic 1

Cribbed from Dragonwarriors.
I've just realised that you can use MAGICAL ATTACK as a d20 roll over save or basis for a DC when converting system-to-system. Not perfect, but simple.

Searchers of the Unknown and Lamentations of the Flame Princess are (approximately) the two games I came up with when trying to make D&D-a-like from memory, at the end of my ttrpg hiatus. And then I learned that those two systems already existed (and had for years) when I found out about the OSR.

Basic 1 is a barely-designed Old School adjacent system, nominally based on a version of D&D and a pre-AD&D Gygaxian house-rule. 

It's also a tool for thinking about this kind of game/system - because I'm a tweaker, and the description applies in both senses. Metaphorically.

The system defaults to vanilla fantasy of the kill-the-monsters-claim-the-treasure type, but adapts as well/badly to other genres and styles as any other system. Wherever there are gaps, it's assumed you will either plug in a pre-existing subsystem or make up your own. It's not meant to be D&D, just made out of some of the bits.

This is also how I came to D&D/rpg in the first place - I had some understanding of systems (mainly from gamebooks and reading White Dwarf) and most of the dice, and then the enthusiasm to create, so I tried to make my own before I'd even opened B for BECMI. Pretend you don't own any other dice and just use d6* - this mini-system is also based on me not having a d20 at the same time as not appreciating that OD&D, B/X, BECMI and AD&D were different things.

Because there's nothing new under the sun, I don't know if this already exists - there is only so much information I can reliably process. Comment and links to fait accompli welcomed.

It gets its name from the Epic 6 (E6) hack for D&D 3e, with the alt from the B in BECMI (though could just as well be B/X). Your characters will never rise above 1st level, but they can spend earned XP to boost their abilities for the next adventure. Your enemies will also mainly be 1 (+/-) HD wonders, with the real monsters having 3 or 6 HD.

Characters.

Make four characters, one each of Cleric, Fighter, Magic User and Thief. 

Roll on some random tables to equip and give them backgrounds: your non-class abilities are derived from/implied by these results - characters can do what you expect. You can also use random starting cash to equip them.

This is your stable and can never have more than four characters in it. Preferably, only take one on an adventure at a time, giving the others time to heal, train and/or do research.

Stablemates can share cash and equipment, within reason, but remember to pay for their upkeep while the others are on adventures or they'll starve, fall sick, get kidnapped, be imprisoned for debt or otherwise make real-life trouble for themselves. They can even have jobs, hobbies and off-camera intrigues.

Characters can only be replaced when they die or retire, and retire means you have to set them up for life, not just dump them and take their stuff. Once you are bringing in replacement characters, you are no longer restricted to one of each and you can also start adding non-humans and from other editions/systems.

Hit Dice.

As per BECMI: Fighters d8, Clerics d6, Magic Users and Thieves d4. 

In Basic 1, HD are also used for hitting with.

You can have max. hp, if you want. 

Unless damage is overwhelming, 0 hp or less means you are a Casualty rather than dead - even wounds in combat or having a house fall on you doesn't need to cause hp damage and could have other effects (death save spiral unless rescued, for instance).

Ability Scores.

Classic Six - Strength, Intelligence, Wisdom, Dexterity, Constitution, Charisma - rolled up using whatever method you can all agree on. 

Ability Score Adjustments and Prime Requisites.

Your available/preferred edition of D&D for adjustments and Prime Requisite XP bonus/penalty, or none at all.

Older editions are less generous; AD&D is more detailed/all over the place, and has Percentile Strength for Fighters.

Doing Stuff with Dice and Numbers.

Make rolls (checks, saves, tests, whatever) against Ability Scores using 3d6 roll under (4d6 or even 5d6, if difficult/unskilled/no tools). You can use standard pre-3e and 3e saves, but I've not included them because they were hidden information when I started out.

Using 2d10 or even d20 isn't forbidden. If you want to use d%, the usual thresholds are stat %, x3 and x5. 

Advantage/disadvantage, more-dice-and-drop-some, doubles/triples and crits can all be applied according to situation and taste. Tunnels & Trolls-style saves also valid. If you like, exploding dice can also be a thing.

Thieves can perform all class skills on 1-2 on d6 - but only need to roll in difficult/high pressure situations. The classic Thief skills are class specialisms, and not exclusive - anyone can give them a go, and (say) a Fighter who was a locksmith might be better at lock-picking than an average thief.

Clerics can Turn Undead on 2d6: 7+ for 1- & 1 HD, 9+ for 1+ HD. Clerics Turn 3 and 6 HD undead only on a double six, and are fatigued by success (this is a gap).

Non-Clerics can Turn on double six, but success fatigues. They have no chance to Turn 3 and 6 HD undead.

Combat.

This section is the longest, not because the combat system is the most important part of the game, but because of the legacy of mechanical density and system interpenetration.

The base for the combat system is human vs. human, hand-to-hand - magic, missiles and monsters (including most animals) are all specials. 

Fighting is opposed rolls, HD vs. HD - highest roll wins. Everyone gets to attack and everyone gets to defend. You can use attack vs. defence, or opposed rolls counting for both at once (see Shields below).

Surprise gives you +2 attack/damage and them -2 defence/0 damage. Initiative/First Strike is a gap.

Anything other than melee back and forth (special techniques, tricks, gambits etc) can be resolved with checks/saves/tests, if you don't think the HD is appropriate - this would normally be instead of an attack, but defence may still be allowed.

Gang up to add your HD together for better attack or defence (one wound only if successful, but +1 damage for each ally above 1/you) - you can attack or defend as a group, not both. Logical limits on how many can gang together.

Damage is d6 - Fighters get advantage, Magic Users get disadvantage. Strength adjusts attack/damage. Two-handed weapons get +1 damage. But attacks don't have to cause hit point damage.

You negotiate the difference between weapons through properties and techniques (eg. more people can gang up if using spears; daggers give advantage on attack if you don't defend; tridents can disarm opponents on tied rolls - these are not canon). 

If you really need crits, roll d20 or d% alongside your regular HD for that elusive 5%.

Thieves can use Dexterity bonus to adjust attack/damage. Thief backstab/ sneak attack/ snipe is +4 attack/damage.

If a Fighter kills a +/- 1 HD opponent, they can immediately make another attack (but no defence roll) if there is a suitable enemy within reach. You cannot do 'bag of rats'.

If you are unarmed vs. an armed opponent, you attack with disadvantage. If you are unarmed vs. an armoured opponent, you roll damage with disadvantage.

Shields.

Shields are +1 to your roll, but if you win because of it, you have blocked the attack rather than struck your foe (if it matters, you have still 'won'). Dexterity does the same, though represents dodging and reflexes.

Shields Will Be Splintered applies, but also after each combat roll d4 (cheap/ improvised/ light/ shoddy) or d6 (quality/ heavy shield) - on a 1, the shield is broken and must be replaced. Could also roll vs. 3 HD and 6 HD monster attacks.

Armour.

Leather/chain/plate (or light/medium/heavy) armour as follows (choose/negotiate):

• soaks 1/2/3 damage
• protects completely vs. rolled damage of 1/1-2/1-3
• cumulative protection vs. 1st/2nd/3rd successful hit per combat

After each combat, roll d6 - on a 1, armour degrades one step until repaired or replaced. Roll d4 if vs. a 3 HD monster. Automatic vs. a 6 HD monster.

Magic.

Magic Users get 2 spells - randomly determine or select from appropriate list. You can build up a spell-book and make scrolls. You can purchase more castings as you accumulate treasure and XP (see below).

Check this previous post for some guidance/ideas on using spells from other systems and without levels.

Clerics roll 2d6 to see if they get a spell whenever they start a new adventure (except the first), on a 7+ they have 1. Randomise, select or designate appropriate to deity and faithful behaviour. They can purchase more castings in the same way as a Magic User.

Thieves can use a scroll if they roll double six on 2d6. Or they can buy temporary knowledge of casting (max. 1 or Int mod, whichever is higher) with their treasure/ XP.

Multiple damage dice spells can be divided to reflect individuals affected (as a 'normal' target had a d6 or d8 HD), or the dice can be the saving throw rolled against the relevant Ability Score (ie. a basic fireball rolls 5d6 to beat the target stat to kill/casualty).

Missiles.

If it seems reasonable in a combat encounter, missile weapons can be handled in the same way as melee.

Otherwise, use a Dexterity check/save/test to determine success or failure. 

Thrown weapons, slings and drawn bows can be adjusted for Strength and/or Dexterity to hit and/or damage as appropriate. Being able to use/being equipped with a bow or sling implies a specific skill set for your character.

Crossbows and firearms can be adjusted for Dexterity to hit, but always count as two-handed weapons for damage. Anyone can pick up and use these projectile weapons (for the purposes of Basic 1).

Monsters.

Most common monsters will have +/- 1 HD. The exact die used can be based on the edition (d6, d8, 3e+ monster type HD etc). This covers anything with +/- HD or less in D&D adjacent material, and is the default for anything approximately humanoid and adult human-sized.

3 HD monsters are the Bears and Ogres and Trolls; 6 HD are the Dragons and Elephants and Giants.

3-6 HD monsters roll all their HD to attack and defend, representing how hard it is for normal humans to just go toe-to-toe with them. They can also have an attack allowing them to use all their HD as damage dice or split between multiple targets. Your mission is always to find a better way so that you don't have to go dice-to-dice - at the very least, you need to gang up and/or grapple/overbear them. 

Use raw numbers from printed HD to give + to hp and combat rolls where a Basic 1 1 HD monster falls into the grey area (ie. Bugbear can be 1 HD +4 hp & combat rolls). 

Give monsters special abilities to reflect why they have high HD RAW when converting - sometimes the answers will already be in the description. Damage caused is less important than the mechanical/ narrative effects (eg. your equipment being set alight is more important/interesting than what dice is rolled for damage RAW).

Monsters have default Ability Scores of 9 + HD for special cases and saves, adjusted as seems appropriate and/or based on their description. Ability Scores don't mean the same for monsters as they do for characters.

Monsters do not have to stick to the same rules as the players, but not as an excuse for unfunny TPKs.

Experience and Treasure.

Lots of gaps here.

You purchase benefits for the next adventure using XP and/or treasure. I'm being deliberately vague on what benefits are, because you might be able to bring something to the narrative/world-building with it - like you purchase an ally who will turn up when needed, a weather event that suits your plans, even a safe place to rest in the dungeon.

Treasure can be spent on any character in the stable, but XP is specific to the character that earned it.

If you aren't planning to use an existing subsystem, awards and costs could be drawn from a pre-existing, repurposed chart:

Search for 'xp budget' or 'xp threshold' to find more charts - other editions, other games, bloggers etc.
Another good one here.

Want to know how much XP surviving the 5th level of the Deadly Dungeon? It's right there.

Want to buy an ability or a spell tied to a high level? Look it up and start negotiating.

I'd make costs cumulative and maybe apply a hard cap of 5 benefits max. or 1 benefit + 1 per previous adventure for each character, but you don't have to.

Buying an ability outside your class will always be more expensive, but I don't see why you shouldn't do so. If you've got access to the material, you can buy Feats if you can make them run within Basic 1; the same with magic items - though they're only good for the next adventure (give them a usage/refresh die rather than charges, and you can buy the same one next adventure and say it's still the original one).

Ability Score advances would be priced according to the relation to your class and the levels they would normally apply (generally every four levels) - cumulative costs, so three advances would be (using the Easy column above) would be 125+450+1,000=1,575 rather than 375.

An alternative is to use something like Spwack's X System for DIE TRYING, which I first encountered in Sofinho's PARIAH.

*If you've only got d6, Fighter HD becomes d6+1 and everyone else d6. The d4 roll for shields vs. 3 HD monsters can become a d6-1 or d3 roll.

Commentary.

I needed a break from thinking and writing about Monster Manual 2, and this is the result - clearing out some of those crowding background thoughts and giving me something to refer back to.

I'm also getting some emerging combat/encounter system ideas in opposition to the rather one-size-fits-all approach of D&D.

It's probably a broken system, but it's more for thinking with than playing with. 

The primary driver behind pulling this thing together was reading this post at Dreaming Dragonslayer about Rogue Runs in Darkest Dungeon (which I've been enjoying as a play-style) and this post at Wodan Gaming about doing away with spell levels (and the linked Zzarchov Kowlaski post). 

Altered State Subsystem (wip) with CW: Drugs

CW: mainly the drugs, but mentions sex, kink and self-harm.

Turns out I need an Altered States sub-system for the Druid (BOSR/ Folk Horror/ Pagan/ Psychedelic/ Witch variant) I'm working on. 

I'm sure there are some already out there, maybe already part of the setting or system you're using. Plus some links for drug-based rules resources.

My unadorned and undeveloped system is this:

• without shortcuts (and I mean drugs), it takes exploding d2, d3 or d6 hours to get yourself into an Altered State
• unless from drugs, it takes d2 to d6 turns to come out of an Altered State; these dice might be exploding if it's a particularly powerful Altered State; what you have to do depends on the Altered State
• in an Altered State, you are -1, -3 or -6 on all Ability Scores depending on how deep you go; penalties may persist afterwards, from fatigue, injury or sickness
• while in an Altered State, give your character sheet to your GM - you may not consult it, take notes or keep written records during this time*
• if you need a dice roll for going in and coming out, I suggest our old friend 2d6 vs. 7, 9 or 11; ability and skill checks would also suffice

The basic idea is that the Altered State, while it has other uses, should carry a penalty of some kind - under the right circumstances, this could just be the time required to achieve, utilise and emerge from it, but I'd consider making the handing over of the character sheet an absolute. 

Ways to get into an Altered State: dancing, chanting, drumming, breathing techniques, meditation, humiliation, sensory deprivation, fasting, over-indulgence, heat, cold, suspension, submersion, asphyxia, tickling, flagellation, sustained vigorous exercise, spanking, masturbation, near death experience, sex, wrestling, fighting, staying awake, prayer, staring at something, crucifixion, fever, disease, concentrating really hard, madness, fear, laughter, frenzy, sadism, pain, flickering lights, acting really convincingly, silence, screaming, singing, counting, extreme mathematics, hanging nine days on the World Ash with a spear in your side, a really long walk.

And drugs...

...whether magickal herbs or magic mushrooms.

There are some rather bland rules for Intoxication in the 1e AD&D DMG, which could be expanded to cover all Altered States if you prefer the system.

Carcosa has its Desert Lotus varieties, with their own defined effects and durations.. Blue, Green and Jale would be the obvious choices, with White Lotus a borderline case (you become a mindless zombie). Black Lotus is fatal, but so are some poisons that have medical uses in small doses.

For drugs (entheogens) as an integral part of the setting/system, you should look to the excellent aloneinthelabyrinth.blogspot.com and check out Pariah - a proto-neolithic psychedelic animist rpg. 

Pariah has flavourful rules for ritual (and recreational, I suppose) use of entheogens, with one of the explicit purposes being travel to other worlds (which are also delicious). They are not just 'magic potions' - they're a session in themself.

Applied to another setting, you might decide to leave out the cosmological aspect - maybe the drugs just aren't as good as they were at the dawn of time, or make them incredibly rare, or set up gatekeepers, or have the common supply be so over-processed that only prodigious consumption (or in its natural/pure form) would have any special effects.

Two drug-use and misuse subsystems, one from Goblin Punch and one from Ten Foot Polemic. 

The Ten Foot Polemic system gave me the idea for handing over your character sheet when in an Altered State, as taking Alterket means you no longer keep track of your hit points, the GM does.

And Goblin Punch has a selection of drugs (including for Dragons) you can choose to tone down for the family audience. Or not.

Just as mechanical and narrative effects of drugs can be used to represent a non-drug Altered State, so can rules for Tolerance (you need more chanting to get in the mood) and Addiction (you won't leave the meditation centre). 

The Ghastly Affair system includes some fairly comprehensive mechanical/narrative details on IRL drugs of the Gothic/Ghastly Age - laudanum, hashish, peyote buttons etc. - though they do include some supernatural side-effects, including being Attractive to Fairies and increasing the effects of Divination.

*there may be individual IRL circumstances where this isn't appropriate - issues of communication, memory and trust, for instance.

Kinder Surprise/Mix-and-Match Magic System(s).

I always wanted a magic system that was stuffed to the gills, but managed to be defined and unified. 

But essentially it boils down to I didn't see why a 1st level BECMI Magic User shouldn't also have a chance to cast summon byakhee.*

This isn't that system, but it is a rambling collection of resources, ideas and suggestions that might be of use. It's sort of system agnostic, scaled to 3-18 Ability Scores and d20 or % mechanics, and I've successfully used 3d6/4d6D1 homebrew mechanics with some of it. 

Source material includes Fighting Fantasy, Call of Cthulhu (2e/5e), Dragonwarriors, iterations of D&D, Tunnels & Trolls 5e, Dragonroar, a little bit of Runequest. 

Here's a summary of a great magic system hack for AD&D by Black Dragon Games/ Dweller of the Forbidden City (Twitter / blog) - it's a better general synopsis of some of the things I want than I can easily articulate:

• Magic Users can cast a number of spells per day equal to their Level, whatever the level of the spell.
• To learn a spell, they must successfully roll their % Chance to Know (Intelligence Table II: Ability for Magic Users, 1st ed. PHB p. 10) or they can never learn it.
• They can learn/know a maximum of (Intelligence + Level) spells.
• To cast a spell successfully, % roll vs. Chance to Know (as above) + 1% per Level (max. 99%).
• The % chance of suffering harm from a failed casting is (Spell Level x5) - caster Level.

The original Twitter thread should be here.

You could boil it down to a d20 Intelligence check adjusted by (Spell Level - Magic User Level), used for learning, casting and possible harm, with natural 20 always failure/harm and natural 1 always a success. 

This is slightly less punitive than the system in 2e Masque of the Red Death, but that has Ravenloft Powers Checks as consequences for casting spells - not just failure. As Powers Checks are %, you can bolt them straight on to your home system if you want them as an addition/alternative.

Another % hack for D&D spell casters is to use the following from 1st ed. DMG, p. 128 (it's originally for scrolls, but you get the idea):

This was also suggested by BDG/DofFC in a Twitter exchange touching on crossing the magic systems of older edition D&D and CoC (seriously: BDG has been a real educator in AD&D, a system I thought I knew better).

Because BRP (more specifically, older edition RQ and CoC) uses %, it is easy/easier to bring spells across into a D&D adjacent using these mechanics. Working out the levels of spells that do not have them could be as simple a matter as saying 'the magic point cost equals the spell level', but you might prefer to convert them more carefully (any spell causing non-permanent characteristic loss could be 3rd level minimum; permanent loss, 5th or 6th).

And, because we're still talking %, you could port the chances of spell failure while wearing (metal) armour for Sorcerers from Dragonwarriors (10/20/30% for ring/chain/plate), either as absolutes or as additional penalty. If you prefer, the progression could be leather/chain/plate, and answers why can't/doesn't a Magic User wear full plate (but has no problem using a shield, it would appear).

DW Mystics risk spell failure on a similar scale if wearing magical armour (10% chance per bonus point), which might also suit some Magic User types (or Clerics, if you play an arcane vs. divine setting). This might also apply to protective magical items, too.

Using Spells From Other Systems.

There's enough points of similarity between D&D, CoC and DW, that you could use spells directly from one system in another without too much change. 

Range, duration, area and general effect are no problem. There should be ample crossover material in systems for fire, acid, poison and so on. The 5% increments on a d20 work in both conversion directions. And subsystems peculiar to an individual spell will sometimes work on their own, outside of their origin system.

As mentioned above, CoC and RQ spells are D&D spell level equal to their magic point cost and DW spells cap at 10. For D&D, anything that would convert to a level greater than 9 can be treated as 9, but I'd go with 10 instead - representing magic that is rare, raw, complex, unfamiliar, alien; these spells are from the equivalent of a different universe, by some metric. Level equals magic point cost if you're going the other way.

CoC spells that cost/damage characteristics (temp or perm) can also do the same to D&D and DW stats, because they're within the 3-18 scale.

POW maps to Charisma, Wisdom and Intelligence in D&D (decide or randomise), and to Psychic Talent in DW (which could mean a character loses their spell-casting ability if they drop below 9 - whether a DW Assassin would lose any or all of their abilities is a question raised).

Spell damage can be taken directly from the spell descriptions, but bear in mind that these will have been calibrated for the game/setting and those expected hp totals (5-18 in CoC and never changing; min. 7 in DW and only +1 per level). D&D spells taken to CoC will be cast at the lowest spell or caster level possible; in DW, you can do the same, or scale them with the caster's rank.

DW flat damage is roughly the average of a D&D dice/die roll, and most DW weapons use the same die for Armour Bypass as they do damage in D&D. Older edition CoC/RQ weapon damage is not far off D&D.

Spells from 2d6 Fighting Fantasy SKILL/STAMINA/LUCK systems can be ported, using their STAMINA cost as magic point cost/ spell level. Bonuses/penalties from spells can be used directly. In the gamebooks, you usually wielded a sword and scored 2 damage per hit, so calculate damage as d6 or d8 for every 2 STAMINA damage (or part of/potential 2).

SKILL maps approximately to rank, Level/HD, to Hit, ATTACK/DEFENCE/EVASION, Dexterity and Intelligence; STAMINA to Strength and Constitution, maybe SIZ, too; LUCK to Charisma, rank, Level/HD, POW.

Some spells have analogues across systems, but should be treated as different spells (representing more or less developed versions, other magical traditions, lost spells, newly invented spells and so on). For instance, the DW Sorcerer spell invisibility is a 9th level spell (9th rank caster) but only 2nd in D&D.

Access to spells from non-D&D systems will give D&D Magic Users significant healing magic, so consider dropping or modding the Cleric class if you go whole hog for this kind of thing.

Intelligence Instead - Doing Away with Spell Levels.

A minimum Intelligence of 9 is necessary to be a Magic User in AD&D (and a Sorcerer in DW; Mysticism doesn't, because it's not 'academically demanding'), and you need a minimum 10 Intelligence to be able to cast a 5th level AD&D spell.

Thus, a spell could be given an Intelligence rating of x2 its level (with the option for 1st to 4th level spells to all have an Intelligence of 9), and this could be used to rate how difficult it is to learn/cast. Percentages would be 100 minus the equivalent chance to know, or based on the difference between spell and caster Intelligence, using the scroll table above.

Another option, using d20 Intelligence checks, would be to generate a bonus/penalty using the AD&D magic jar subsystem, or each 4 or part of 4 points of difference generates a +/- 1 mod to the roll, or some other match modifier method.

This obviously makes higher Intelligence Magic Users significantly better at learning and casting spells, but I tend to think of Intelligence as your character's Magic User-ness, rather than the intellectual and academic capacity of the character - player skill is, within some limits, character skill.

Use the caster level to calculate spell effect; negotiate or rule on whether the minimums in the spell descriptions apply under this regime.

If you're doing away with levels altogether, spell level for effect can be:

• 1st level
• minimum level of the spell cast
• caster level is equal to half INT (powerful)
• caster level is 1 + Ability Score bonus (as appropriate)
• 1 + how many STR you are willing to sacrifice (recovery rate to be determined; you can kill yourself doing this; other Ability Scores can substitute as appropriate)
• burn 1 hp to get the spell going, then hp equal to the casting level you want
• burn hp equal to the spell level, then +1 hp for every level you want to boost it by

Both hp burn and STR methods open up the possibility of others contributing, either by taking part in a ritual casting or as blood sacrifices.

Spells for Unlevelled Casters.

Whether you mean 0-level NPCs in a levelled game, or because you're not using character levels to measure experience, or because the caster is not a Magic User by class, they can cast daily spells equal to the relevant Ability Score bonus (or total, or average, of CHA/INT/WIS) - the amount will depend heavily on which edition of (say) D&D you use to calculate your bonuses.

In fact, you might want to extend these bonus spells to regular Magic Users, but I'd rule that these are front-loaded and not cumulative with those gained by level.

The Spellcasting Day.

If you follow spell memorisation RAW from AD&D (can't speak for the detail of later editions), you don't just wake up fresh and full of spells, or get up and leaf through your travelling spellbook over iron rations - you might not ever get to fill all your memorisation slots because you don't have enough time in the day. If you want to memorise a single 9th level spell, you need 12 hours rest/sleep and then 2 hours 15 minutes of meditation/study for that spell alone.

Dragonwarriors brings a more flavourful question of timing (dramatic and practical) to spell recovery. It's a magic points rather than spell slot system, and spell-casters know all spells of their level, but it works if you're not strictly Vancian. 

Sorcerers regain their spells/ magic points at midnight (precisely, apparently, so you can presumably set your clock by one). Mystics recover at dawn and Warlocks replenish their reserves at sunset - more nebulous and debatable. Are they connected to the sun, its light or absence? What about in the extreme north/south of Earth-like worlds?

DW Elementalists must perform rituals under specific conditions to regenerate their casting ability, opening a whole host of options/restrictions for casting traditions - sacrifices, lunar cycles, your horoscope, eclipses, astral conjunctions, lightning strikes, retreat to inner sanctums etc.

Duration.

Some alternatives to minutes/rounds/turns scaling to caster level:

• one battle: presupposes a combat spell; no chance of expiry mid-combat may take some of the drama out of things
• one encounter: not necessarily a combat spell, and can be very brief or an extended period, and depends very much on how you define an encounter
• as long as it needs/takes: utility spells, applied to the task at hand; can include spells that require a trigger to discharge
• concentration: at minimum, should be disrupted by casting another spell; injury is not necessarily going to spoil concentration (spell maintenance as serious discipline), but will likely be the first choice of opponents
• instantaneous: the bang-and-done spells

A spell duration of 'a day' should be more like the spell-casting day than a 24 hour countdown. Or measured as until you go to sleep (including KOs and sleep spells) or until you wake up - variant healing spells that demand a good night's sleep as part of the deal, or extraplanar communication, or go to sleep, wake up somewhere else.

Permanent spells might not be literally permanent, but practically so for game purposes: 7 days, a lunar month, 40 days and 40 nights, a season, a year-and-a-day, 666 days, 100 years.

Duration to when a condition is met or broken is a feature of folklore and myth, and works for all types of spells (except maybe the instantaneous ones, but even then there might be some room for exploration).

Spell Expiry Roll.

Lifted straight from Dragonwarriors: roll 2d6 at the beginning of each (6 second) combat round, on a 12 (or 2, if you prefer low rolls bad) the spell ends.

According to this source, a per minute Spell Expiry Roll would be 25% expiration chance each time.

(There's also this thread on predetermining DW spell durations, but I'm inclined to think that if you're going to do this, you don't need the mechanic in the first place)

For DW Warlocks, the Spell Expiry Roll is made on d20, the spell ending on a 20 (or a 1, if you prefer); with a 40% expiration chance minute-to-minute.

Or it could be an Intelligence check each round (or other unit of duration), or Strength (nod to Tunnels & Trolls).

Or cumulative rolls -on whatever dice you like- until the total exceeds your Intelligence, or your Strength, or the average/total of your spell-casting characteristics, or any of these plus your Magic User level.

Fatigue from Casting.

As much as I like the Psychic Fatigue mechanic from Dragonwarriors (roll equal or less than 13 + caster level - spell level or you can't cast another spell until the next dawn; 20 is always a fail), I haven't found a simple way to make it compatible with an/the envisaged unified magic system - one of its purposes is to mark a clear line between the magical technique/ tradition of Mystics and Sorcerers. [edit: I did, later]

However, actual physical fatigue related to spell-casting is baked into multiple systems (and much of fantasy/magical media via IRL beliefs, traditions and examples): Dragonroar spells use up Endurance, FF uses STAMINA, older edition T&T uses STR, going to 0 magic points in CoC knocks you unconscious.

And this bit (AD&D 1st ed. DMG, p.40), giving one of the reasons 'why Vancian?':

Using the BDG/DotFC system (for example), a level of fatigue could be the harm from a failed casting.

A simple fatigue progression for spell-casters I use (and for Light/Serious/Critical Wounds) is Tired/ Exhausted/ Spent, with non-cumulative penalties to all Ability Scores of -1, -3, then -6. Once Spent, you would immediately be placed in the out-of-action-but-not-dead-yet condition for your game if you took another level. And why not let the one after that be fatal?

Full or partial recovery from fatigue and full recovery of spell-casting capability are not necessarily linked.

Casting Beyond Your Limits.

I've not playtested this, because it's not come up, but the Magic Users I'm thinking of can cast their daily complement of spells (1 per level, or 1+level) without penalty - they can then attempt to cast spells beyond this, with the risk of failure and harm.

Either use BDG/DotFC % (if you're already using it, you could apply a penalty), or an INT check for casting and a CON (or STR) check (whether or not the casting succeeds) to see if you are drained and take a level of fatigue (see above).

Armour penalties (1 for every AC/armour point better than Unarmoured) would apply to these rolls - even if armour has no effect on normal casting - giving a mechanical reason for the legacy inclusion of unarmoured Magic Users (and something like a legacy Cleric, who wears heavy armour but chooses not to risk failure/fatigue once their daily spells are cast).

If you are trying to shoehorn Psychic Fatigue into a magic system that's already quite full (see above), a failed INT check/casting could indicate Psychic Fatigue - you cannot attempt to cast any more spells until your usual recovery condition/time, and you still take a CON/STR check for physical fatigue.

Option vs. Spamming.

If you try to cast a spell a second time in a spell-casting day, it is treated as if you were casting beyond your limits - you must roll to cast, and roll for fatigue; it also consumes a spell-slot.

Use this option to encourage variety and creative spell use/ to punish system mastery, depending on your point of view.

This option harks back to the little bit of Gygax earlier - you've used your Vancian privilege, and now you've got to pay a price. Imagine the spell as a brand on your spirit, repeat castings pressing it deeper into your sizzling ectoplasm.

A less punitive alternative would be to allow repeat casting (using spell slots as normal), but requiring a roll for Psychic Fatigue relating only to that specific spell - which is why it's worth knowing the D&D invisibility spell (for example) as well as the DW one.

IRL wild talents often claim to have faked phenomena (while still claiming the reality of previous events) because of the demand for reproducible, measurable effects. Whether you believe this or not, let's say that magic doesn't care about the scientific method. I think the same might be true in magically saturated settings, too.

What Was Left Out.

Lots of stuff: The Beholder Contracts (Great Wizards), Lankhmar (Black and White Wizards), Maelstrom (free-form magick), A Ghastly Affair (incantations, pacts, talismans, rituals etc.), A Mighty Fortress (the Scholarly Mage kit and bad weather with spell-casting), Grey Star the Wizard (magical disciplines, rather than spells), Runequest's division of magic into spirit, divine and sorcery.

Some of it might pop up later.

*I know LotFP effectively did this by making summon a 1st level spell, but as a kid in the 80s I couldn't be sure Gygax wouldn't come round and administer a punishment beating.