Compiled Conversion Methods 1: BRP & DW to D&D Adjacent.

Converting To D&D Adjacent...

...From Basic Role Playing (Call of Cthulhu, Runequest, d100 systems).

Basic Method.

This is the one used for gods and monsters from Call of Cthulhu (BRP), given in Cthulhu d20. 

Use one or the other, or both; I don't see why combat ability should be absolutely tied to Hit Dice.

Also, expect lots of human-sized monsters ending up in the same Hit Dice/hit point bracket, due to the way BRP hp are calculated (average of CON & SIZ).

• Divide CoC hit points by 5 = Hit Dice
• Divide highest % 'normal' attack by 10 = + to Hit/ BAB
• Hit Dice can be extrapolated from the + to Hit (count on from chance to hit an unarmoured target), instead of using the hp calculation.

Further Elaboration.

• Armour Class: if it doesn't seem utterly ridiculous, convert BRP Armour Points to AC bonus. 
• Anything with a high average DEX could have additional AC bonus, based on normal Ability Score adjustments.
• Lots of CoC monsters have resistance or immunity to certain types of damage; these can usually be ported directly.
• Using the D&D adjacent Troll as the base monster for measuring regeneration, some CoC monsters might seem to have quite high rates, but these can also be ported directly.
• Movement: in CoC 5e, a Normal Human has a Move of 8 arbitrary units of distance per round, use that as your base for calculating D&D adjacent Movement - for example: a CoC Move of 12 would be equal to the Movement of a Normal Human + 50% (or x1.5).
• RQIII has different, more involved movement rules, and Normal Humans have a base Move of 3 (approx. 30 metres/ 100' per combat round), but you could use a similar % or decimal conversion method.
• Number of Attacks: this is usually in the monster description (ie. a CoC Ghoul has Claw and Bite in its statblock, but that it uses claw/claw/bite in a round is in the text).
• Damage: for 'smaller' CoC/RQ monsters, especially those of roughly human-size, you can just use the dice given (ignore the Damage Bonus, or convert to d4=+1, d6=+2, anything above is +3 or more).
• Special Attacks: poison, paralysis and spell-like abilities map easily onto D&D adjacent saves/ Ability Scores, or may resemble a pre-existing spell or special attack.
• POW vs. POW/ magic points vs. magic points: save vs. spells or paralysis, as appropriate, or an INT/WIS check.
• STR/SIZ vs. STR/SIZ: probably a swallow, grapple or trample; could also use a save vs. petrification.
• Attacks vs. CON, including poison, can use a CON check or poison save, if you're not sure.
• Intelligence: use the average INT - tweak if you think it needs it.
• Morale: no real equivalent; you'll have to work it out from any behavioural description and your own concept for the monster.

...From Dragonwarriors.

Basic Method.

Mainly my own effort, relying on the common feature of d20 Hit Rolls and accidental/designed similarities between the two systems, and covered more fully in a previous post.

You can work out HD from BAB or from Health Points, or you can keep combat ability separate - this seemed particularly apt for the Zombie, hard to kill but a poor fighter.

• (DW Monster ATT - DW Normal Human DEF of 5) - 6 = + to Hit/BAB.
• Hit Dice can be extrapolated from the + to Hit (count on from chance to hit an unarmoured target), instead of using the hp calculation.
• Max. Health Points divided by 4.5 = Hit Dice.
• this will likely be a decimal, so round to the nearest whole number, or use all (or some) of the remainder as a + to hp.
• AC bonus = DW Armour Factor.
• For damage, either use the Armour Bypass roll or x2 the fixed damage to work out the damage die to use (2=d4, 3=d6 etc.), as you feel appropriate for your setting/the challenge you want to present.

Further Elaboration.

Not everything here was covered in the previous post, but arose from later conversions.

• If the difference between the monster's ATT and DEF is less than 6 (usually due to flight, but not always), give it a further AC bonus (possibly +1 for every 2 or part of 2 points less than 6).
• A +1 or +2 Armour Bypass/damage bonus in DW indicates 16-18 and 19 Strength respectively, and either the bonus or the Strength score can be ported directly (DW attributes are on a 3-18 scale).
• For special attacks causing damage, divide max. damage by 4.5 to get number of dice (choose appropriate type) to roll.
• Elementalist, Mystic, Sorcerer, or Warlock levels can convert directly to Cleric or Magic User levels, and spell ability (this is not entirely accurate, but is easy).
• Anything with a MAGICAL ATTACK score can attack/save as a Magic User or Cleric of HD equivalent level, if that is more advantageous or suits your concept of the monster. 
• You could also measure the score against a DW spell-caster to get a Rank/level equivalent if you prefer.
• For Movement, I'd compare it to a Normal Human, as above (or to a Bat, for flying things).
• Out of the Shadows gives details of how able the monster is to see in the dark and how it will react to bright light.
• Intelligence and Morale will have to be extrapolated from the descriptions.