Steps Away From Doom - Some Survival Horror Mechanisms.

You'll Never Leave.

For some reason, re-reading the 1st edition Monster Manual 2 got me thinking about survival horror - in particular reality-bending Silent Hill-style survival horror (but not exclusively). Even Basic 1 and an incomplete post on combat with Bears and Whales seem to be a strand in this thinking.

I'm pre-supposing low or zero-level characters (older edition D&D and retroclones/ adjacents are sometimes presented as survival horror exemplars), and at least a passing familiarity with the genre (conventions across media). But (eg) Predator, Aliens and Resident Evil tell us there's plenty of room for heavily-armed and experienced characters - they all die the same.

And that's where I'll start: you can play survival horror characters as extremely squishy, but there should to be some kind of plot-armour for them because they need to be able to sustain play - the extremes of survival horror favour the one-shot and the contained scenario, rather than a longer campaign (opinion; you might think differently).

Hit Points.

You do not have many; you certainly don't have enough to go around tackling the survival horror scenario like it was a session of D&D. 

If you want to boost survivability via hit points, you could do worse than consider using hp=CON or the average of STR + CON.  Or temporarily adopt a SIZ (Size) Ability Score from Call of Cthulhu for hp=average of CON & SIZ, rather than favour the Fighter-types.

0 hp or less is Casualty state, rather than death state - hopefully, the monsters will not ravage your recumbent form. Maybe decide on a threshold that means definitely killed by damage. If you're using CoC CON+SIZ/2 then 3 hp is Casualty state/KO, 0 is death.

Another alternative is all monster hits are HTC (hits to concuss/ subdual/ non-lethal damage), except on a crit (or odds/evens for more deadly foes) - in this case, consider making PC attacks non-lethal and only contributing to temporarily driving a monster away.

Death's Door from Darkest Dungeon.

At 0 hp, you're at Death's Dooor and have to make a save vs. death every time you take any more damage. This kinda sorta makes you immune to damage until you die or recover, as long as the dice fall in your favour.

According to the wiki, the base chance of death 33%, which is close enough to 1-2 on a d6 to feel like an old school system-related design choice. But you could also use STR or CON or CHA, or even Luck (see below), or however 5e death saves work (I know they exist - it's just never come up when I've played).

While at Death's Door, you suffer significant penalties. These could be ported straight over from Darkest Dungeon, or something like a Lasting Wound or Death & Dismemberment condition.

Fear/ Misery/ Morale/ Sanity/ Stress.

I've already written a post outlining a Misery/Morale subsystem as an alternative to SAN, and in it made reference to other systems (Mothership, Best Left Buried, Call of Cthulhu, Fighting Fantasy) that could be plugged into another. 

Characters can start out with 0 Misery, assuming they don't have a tortured backstory that has brought them into a survival horror scenario - in which case, treat as common adventurers (2 Misery). Relief/ Suffering might need to be modified for survival horror one-shots, as they were initially intended for on-going play.

I recently saw this post, which uses LotFP (so B/X or any other D&D-adjacent) to play CoC, porting over the d% sanity and magic point spell system directly - so simple that I/we didn't think to do it BITD.

If you're using SAN (or something similar), consider using it as another damage track rather than RAW - some of the 'traps' in survival horror video games are scares rather than save or die. SAN works quite  well as those abstracted hit points representing anything other than flesh, blood and bone.

Heart Attacks from Darkest Dungeon.

If you hit 12 Misery or your FEAR/SAN threshold, why not suffer a heart attack?

You might be entitled to a saving throw, or it might just immediately reduce you to 0 hp - leaving you at Death's Door. It definitely counts as 'more damage' if you are already at Death's Door.

If you survive, your FEAR/Misery/SAN recovers somewhat, but you suffer penalties thereafter - a second heart attack in the same session will probably kill you. But it's not just Dr. Who and Grobbendonk who can survive multiple heart attacks (#BOSR).

Luck.

Port over pretty much unchanged from Fighting Fantasy/ Troika!. Will also work as a 3d6 Ability Score.

Luck runs out, so follow Fighting Fantasy convention and deplete by 1 each time you check/ save/  test, whether by choice or compulsion - I'm thinking of it here as being a supplement to survivability without directly boosting hit points (it falls into the category of those abstract hit points mentioned above).

Troika! has you recover 2d6 Luck for 8 hours of rest. For a one-shot or time-limited scenario or if you simply want to be more granular, this serves as a starting point for scaling down. 

You can use it as in T&T: roll over (20/25/etc - Luck) on 2d6, keep doubles and roll again. You can use it as in Fighting Fantasy, roll equal or under on 2d6/3d6/even d20. If you're using the roll under method, doubles and triples might indicate more significant failure/success.

It might be that you can spend an extra Luck point to use a relevant Ability Score bonus, or a total of 3 to use an Ability Score penalty as a bonus. I don't know where this leaves average characters - maybe they don't get this opportunity because they're average.

In T&T and Fighting Fantasy, Luck is the primary saving throw and could be used as a reroll for other failed saves - because it costs to roll it, it isn't a get-out-of-jail-free card, but it does shift survival in favour of the PCs while also placing a cost on it.

Encumbrance.

Survival horror games are also about inventory management, so making Encumbrance/ item slots the hit point alternative or simply making them vulnerable seems to be thematically appropriate.

Damage could either completely remove a slot (you can no longer carry that many items) or reduce slots allowed before suffering penalties.

Death can be at 0 items, or on the next damage suffered. 

I prefer slots to weights, and for a one-shot you could probably go as low as 8 items + 2 weapons (Lone Wolf), 10 items (Fighting Fantasy, Dragon Warriors), 5 items (plucked from nowhere) or 2 (Knightmare, I think).

Can't imagine that this isn't already an established mechanic, but haven't read widely enough to confirm.

Narrative Harm.

Either because you've had your hit points exhausted, or the dice rolls indicate you've been wounded or terrorised, you suffer significant harm - this might be physical, psychological or tied into your capitalised Fate. 

For instance, cannot grip a weapon or crumbling ledge without eye-watering shooting pains; if you see the monster again, you'll reflexively close your eyes and cover your ears; next time, the monster concentrates its malevolence on you. 

Mechanical effects can be decided at conception, introduced as relevant or might not be necessary for more narratively weighted play - you might not be using hit points, or even dice.

Once you've taken harm, reroll your hit points and keep on surviving. Eventually, it will be obvious that you're too damaged not to declare the harm as fatal.

This is a riff on the harm subsystem of Green Dawn Mall by Come Martin. 

It Was A Dream.

Each time you 'die' or are about to, you wake up and go again. 

This implies a certain type of scenario - the supernatural end of survival horror - and should be integrated into the construction of the adventure. It might be the way you get round certain obstacles or short-circuit certain nefarious plans.

Does everything reset when you wake up? Do things remain as they were when you 'died', or do things get a little more twisted? Has time passed at all? What do you remember?

You Were Already Dead and Now You Know It.

A logical progression of the above, and also a genre staple, but how are you going to handle it?

• As far you know, it was all a dream but eventually you're going to catch on.
• You respawn, reincarnate or possess a convenient body. 
• You are now undead, with the benefits and the drawbacks. 
• You are shifted into a parallel dimension/otherworld to continue your adventure/suffering. 
• You are now the spirit guide of those left alive and embodied.
• You are an unseen, voiceless, disembodied presence, cursed to witness bad decisions and gruesome demises - you may get more company, you might not.
• You start a new adventure on the edges of the afterlife, possibly moving from survival horror to the Jim Henson watercolour steampunk of 2e AD&D Planescape (but more likely another dimension of horror).
• Nothing changes.

I Said: You'll Never Leave.