AD&D Gelatinous Cube for Call of Cthulhu 5e

Ten foot of near-invisible, corrosive jelly seems pretty Mythos to me. Statblock after the rambling.

Conversion.

Stats from 1e Monster Manual: AC (descending) 8; Move 6; HD 4; 1 attack @ 2-8 + paralysis; surprise 1-3 on d6; Intelligence Non-(0); Size L (10' cube)

Resistant to cold damage; immune to electricity, fear, hold, paralysis, polymorph, and sleep.

Method.

I'm using a combination of this AD&D-to-RQ document, some of my own Wisdom, and referring back to the 3e Runequest Monsters book (in particular, the Gorp - the closest thing to a Gelatinous Cube in there).

Strength: max. damage of strongest attack (8) + 6; a species average of 14; divide by 3.5 and we get 4d6 for Gelatinous Cube STR. This might be right for a later edition Cube, with its pseudopod.

However, the Gorp lacks STR, and there is good reason to think the same of the Cube: this means it is an incomplete creature in RQ, and having no STR, will never tire.

Constitution: HD + 10; a species average of 14; 4d6 again. Why not?

Size: two possible methods:

• (source) if a 10' x 10' x 10' Cube weighs 15, 000 lbs (or 7.5 US tons), then it has a SIZ of 64 (5e Mythos Comparative Sizes and 3e Advanced Runequest SIZ Equivalency tables) and is about the same as the CoC Elephant or a Great Race cone-being. This is not the Cube I'm looking for, but maybe you'd prefer it.
• it's a Large monster (10' tall), so that's 18-36 according to the conversion document, but only taking into account height/length (if you want to treat it as Huge for its volume, that's 37-61 SIZ).

Intelligence: Non-intelligent, so 0, and is unaffected by emotion-influencing spells (or anything else that needs to act on INT to work). 

Power: half HD + 9; species average of 11 (slightly better than a Normal Human). I don't think this is appropriate, so it should either get the same as the Gorp (3d6), or it should lack POW altogether (incomplete creature; soulless, which seems right).

Dexterity: there is a good reason not to use the 10.5 average as DEX for the Cube, so it has no DEX - just like the Gorp. It cannot manipulate objects, it may only use its natural weapons. While RQ says it can't use stealth, that is going to be one of its special abilities.

Hit Points: the average of CON and SIZ, so:

• (64+14)/2 = 39
• L (median 27) = (27+14)/2 = 20.5 (20-21)
• H (median 49) = (49+14)/2 = 63

Damage Bonus: normally calculated using STR + SIZ. Even just using SIZ, we get:

• 64 = +3d6
• 27 = +1d4
• 49 = +2d6

Some CoC monsters (notably the Shoggoth) use their Damage Bonus as their attack damage, so this is what we'll use for the corrosive touch of the Cube.

Move: 4 arbitrary units of distance per combat round (a Normal Human moves 8). But remember it never tires.

Armour Points: 2, under the conversion document method, but I think the Cube will probably resist physical damage in a more Mythos fashion.

Attack: no need to work this out, it's effectively 100% (like the Gorp); it's not so much it attacks you, as you need to avoid it. This also fits well with the attack forms of numerous Mythos monsters.

The anaesthetic/paralytic effect of the Cube is handled as a CON vs. CON Resistance Roll.

Skills: Hide/Sneak at 60% each (basic 10% + 50% for surprising 1-3 on d6).

Gelatinous Cube, Stealthy Corrosive Mobile Jelly.

Obedient and mindless, the Gelatinous Cube serves as the cleanser and pest control of the echoing, empty, ancient cities of alien intelligences.

All that remains after their masters/creators left/perished, are they alive or are they technology?

STR n/a     CON 4d6 (av. 14)     SIZ 49     INT n/a     POW n/a     DEX n/a     Hit Points av. 63

Damage Bonus: +2d6

Weapons: 

• Touch 100%; outrun, Dodge or Jump to avoid; damage 2d6 + CON vs. CON or paralysis 5-20 rounds.
• Paralysed victims are absorbed and dissolved (2d6/round+suffocation).
• Even if you are not paralysed, you can only attempt to Dodge or Jump on your next action - you take damage and must resist paralysis regardless of whether you succeed or not.
• Paralysed and absorbed victims can be rescued, but rescuers will be unable to Dodge or Jump while doing so.
• All damage is acid damage; Cubes can only dissolve conventional organic matter.

Armour: 

• None, but takes minimum damage from physical attacks and is immune to all impaling weapons, including firearms.
• Cannot be harmed by electricity; extreme cold reduces Move to 2 and gives 15% bonus to Dodge/Jump rolls to avoid, but causes no damage.
• Normal damage from fire and magic (magic weapons cause minimum damage for their type, plus any relevant magic bonus).

Skills: Hide/Sneak 60% - Gelatinous Cubes do not actively use these skills; this represents how difficult they are to notice, even if you know what you're looking for.

And because this is CoC 5e: Glisten Menacingly 75%

Sanity Loss: 0/d4 to see a Gelatinous Cube; 1/d6+1 to see a fellow human dissolved by a Cube; 0/d10 for being absorbed and surviving.