1st Flavour of Skeleton: Call of Cthulhu 5e & 2e Conversion to D&D Adjacent.

Skeletons seem appropriate right now, and anyway I'm a fan.

The HD conversions below are probably more useful for old school settings; the BAB conversions make for a much tougher monster in the lower bonus game.

Image: Retro Horror Top Trumps Skeleton Card
A Skeleton holding a hangman's noose.

Image pasted from Hypnogoria's Tomb of the Trumps, which is worth checking out if you either held on to your original packs (I'm glad I did, plus my sister got me a reissue for Christmas) or regret ever letting them go.

Call of Cthulhu 5e.

Skeletons are statistically close to Normal Humans in CoC, though rolling their SIZ on 3d6 not 2d6+6 and having a fixed POW of 1. They have no CON.

Strength 3d6, Size 3d6, Intelligence 3d6, Power 1, Dexterity 3d6. 

The lower possible SIZ implies non-human (animal?) Skeletons and/or animated fragments (bony crawling hands, flying/rolling skulls, inchworm vertebrae etc.).

The single point of POW is their animating force, but within the system means they are vulnerable to magical attacks (POW vs. POW). Even though SAN and Luck are not normally applied to monsters, these Skeletons are neither Sane nor Lucky.

Lacking CON, Skeletons are mechanically immune to poison, drowning and so on. As hit points are normally (CON+SIZ)/2, I'll say they have 10 hit points for conversion purposes (but see below).

Average hit probability is 31.5% (DEX x3) - better than a Deep One, Ghoul, Mi-Go, Nightgaunt or Moonbeast.

Conversion.

The simplest conversion method I found is from Cthulhu d20 - % to d20 and close enough for what I'm after: CoC hp/5= Hit Dice, and highest attack %/10=BAB.

This gives a 2 HD Skeleton, and/or a  +3 BAB, which is 1+ HD (count from lowest to hit AC 10) or 2 to 3+ HD (count from 1 HD to hit AC 10) in AD&D and 3+ to 4 in Mentzer).

Abilities.

• no hit points so take no damage, but are destroyed/shattered % chance = damage rolled x4
• impaling/weapons half as effective (or only on a crit, but at regular damage)
• immune to poison, drowning/suffocation, KO
• Clatter Ominously (45%): NPCs of equal or less HD must make a Morale check
• Rise Unexpectedly (60%): either
• surprise 1-3 on d6, or
• reform 1-3 on d6
• optionally, they could save at -4 vs. all magic (because of their v. low POW)

Intelligence & Dexterity.

You can play these Skeletons like any other mindless, clumsy undead, but they've got 10.5 INT and DEX on the statblock, and could have everything this implies:

• understanding multiple languages (whether they are physically able to talk or not)
• possessing specialist learning (sages/experts)
• casting spells from scrolls and using magic items (1 POW wouldn't convert to an actual Magic-User, but there's no reason they couldn't use pre-loaded stuff)
• fine motor skills (archers, locksmiths, pick-pockets, crafters, scribes)

d6 Flavours of Skeleton:

1. Tomb Robber: what you do, but in reverse; emerge from their sepulchres to execute planned heists/ bloody banditry.
2. Bone Imp: an undead Pixie or Sprite (CoC 2e: STR 2d4, SIZ 1d6, DEX 4d6 = D&D Strength penalty, Dexterity bonus, lower HD or hp per die). Probably lose any natural magical powers/ ability to fly, but get a whole new perspective on the ceaseless, centuried Pixie-Sprite war.
3. Dry Bones: the animating force is in every piece of the Skeleton, and it is able to discorporate and reform at will, sending bits off on sinister little errands. It can also recover from destruction several times (suggest 1 per HD + 1). Notoriously vengeful.
4. Animated Skull: (d6) rolls along; carried about; too much like a demi-lich for comfort (appearance, powers and/or behaviour); shadowy wings sprout from temples; attaches to a recently beheaded corpse; happy to just sit there and chat.
5. My Pet Skeleton: (d6) bird; cat; dog; monkey; serpent; deer/goat. Behaves much like you'd expect.
6. Revenant Adventurer: either seeking to avenge their death, or has found that doing so did not give them blessed release. Has the abilities of their class-in-life. Very likely favours concealing robes, masks, gloves, big scarves, and/or full suits of armour with closed helms.

Call of Cthulhu 2e.

The Skeleton is in A Sourcebook for the 1920s and suffers from some typos. 

(Of peripheral interest is that several statblocks include CHA instead of APP, showing the link back to D&D via the Perrin Conventions)

Specifically, the Skeleton as written has POW 3d6, but in the Zombie description later we read:

'Like skeletons, the 1 point of POW motivates the entity.'

Which I think is clear enough.

It also lacks INT; clearly intentional. From the description:

'Skeletons need triggering to be animated... Once set in motion, skeletons fight to the death.'

Unless it's also a typo, 2e Skeletons get 2d6+6 DEX (average 13), making them even more Harryhausen than 5e CoC.

Given 9 Hit Points, but they can only be harmed through the same sub-system as affects the 5e version.

They are equipped with a variety of melee weapons, have 1d6 armour points and their habitat is 'anywhere that magicians have worked.'

They're also the only monster in the sourcebook to get a 'Number Appearing' statline: 3d6.

Conversion.

• 9hp/5 = 1.8 HD (1, 1+4, 1+8 or 2 HD)
• Best attack (DEX x 4=52%)/10 = 5.2 (+5 BAB)

Abilities.

• destroyed/shattered (as above), but half-chance for impaling/piercing weapons (or also as above)
• nothing else is explicitly mentioned in the description or stats, but everything for 5e could be applied here.
• these Skeletons are armed and armoured and encountered in groups: not really abilities, but the certainty of armour puts them apart from other system basic Skeletons.

d6 Flavours of Skeleton:

1. Skeleton Army: a military unit waiting patiently for the Triumph of Death; fully kitted out and looking to recruit if you start messing around with them and their stuff. Otherwise, just still, silent, eerie and intimidating.
2. Fang Warriors: sprout from the enchanted teeth of monsters; they may have an affinity with the source creature. Animated by Ray Harryhausen, of course.
3. Exoskeletons: undead giant arthropods; think of them as spooky robots; lose many of their natural abilities (eg. venom, silk, stingers). Natural armour.
4. Flaming Skull: when activated, bursts into flame and flies at you; at least as likely to set a fire as a flaming torch, but the flames are supernatural so things could be worse. No armour.
5. Skeleton Archers: two shots per round; they're as good as, if not better than, a Normal Human with a bow.
6. Drowners: weed-draped and spongy-boned, saturated and heavy with water (fresh or salt); attack is grapple and hold under water. Only destroyed/shattered on a x3 due to being less brittle; impale/piercing weapons still x2. They can swim if unarmoured.

Commentary.

This was originally going to be a much bigger single post, converting Skeletons to D&D adjacent from the systems I have available. But because of COVID-19, I have the questionable leisure to break it up into smaller pieces and put it out more frequently.

Tom Moldvay's article 'The Ungrateful Dead' (Dragon Magazine #138) is very much worth the read, and had a significant influence on how I think about the undead.

Eileen Lucas's 'The End of the World' from the same issue is also a good read, though whether you're as keen to have a global pandemic as your campaign background as you were at the beginning of the year, I couldn't possibly say.