|Iain McCaig 1984|
Armour Class +4 Hit Dice 7 Move n/a Morale n/a
Bloated and limbless, it eternally wallows in the pool of slime it was spawned in.
The Allansian Bloodbeast is an Invulnerable Monster and regenerates 3 hp/round from any source.
It is immune to acid and normal fire. Very hot/large and magical fires (causing 10+ hp damage) cause half damage but it retreats to the bottom of its slime pool for at least 1 combat round.
Electricity causes only 1 point per die, but reduces its attacks by 50% for 1 combat round. It will 50% retreat to the bottom of its slime pool for the duration instead.
Tongue: Its 10' prehensile tongue will grab a victim of size M or smaller and drag them to be bitten or (50%) into the slime pool on the next round. To resist the drag, grapple vs. STR 18.
The tongue can be severed with 6 points of slashing damage in a single round. If this happens, the Bloodbeast retreats to the bottom of its slime pool and will not emerge until the tongue regenerates (1 on d6 on the first day, 1-2 on the second and so on).
Bite: It bites for 2d6 hits. On a 9+, you lose a limb. On a 12, it crushes your skull/bites off your head.
Only those grabbed by the tongue or attempting to attack its weak spot can be bitten.
Slime Pool: If unprotected and susceptible, you must save vs. fumes each round or be nauseated (as Troglodyte stench) and partially blinded (as if trying to avoid a gaze attack).
On a critical fail, you are overcome and collapse helpless. The Bloodbeast will grab you with its tongue at the next opportunity and drag you into its pool.
Anyone dragged into the slime pool suffers d6 acid damage each round and is unconscious after 2 rounds.
Weak Spot - Real Eyes: The Bloodbeast has hundreds of false eyes that burst from blisters on its head. Its vulnerable real eyes are hidden amongst these.
A crit against the head will hit a real eye (or 5% chance). If the attacker knows the Bloodbeast's weak spot, they can increase their chance to 10% equivalent. If this seems too much of a challenge, apply a -4/20% penalty to hit instead (called shot equivalent).
Striking an eye causes the Bloodbeast to suffer half its HD in damage (3d6+3 or 3d8+4, and instead of weapon damage). It will retreat to the bottom of its slime pool to recover at the next opportunity.
I sometimes forget how awesome the Bloodbeast from Deathtrap Dungeon is/was.
IIRC correctly, in the gamebook you can't fight it normally and must either get a lucky strike against an eye, sever its tongue or dodge past it. Or die, of course.
It's not a monster you're meant to kill. However, Fighting Fantasy as a setting tends to the lower end of magic weapon allowance so you might need to go the whole hog and make it like some iterations of the Hydra so it can't be killed with body hits alone. Or go the Call of Cthulhu route and reducing it to 0 hp merely drives it the bottom of its pit.
The cover illustration suggests it has a stinger, but this is not mentioned in the gamebook or in Out of the Pit. But it does get used in the Playstation game, in which it also gets to spit poison, use magic, is vulnerable to the Venom Sword, and has only one real eye.
No idea how the Bloodbeast is meant to have evolved or how it breeds or how it even gets about. It seems likely that it secretes its own slime, as it uses it as a pre-digester.
Maybe that stinger is an ovipositor and implanted eggs are carried away by adventurers and wandering monsters?
Mechanically, used some bits of the (AD&D) Froghemoth and the (Dark Sun) Fael and stuck them onto a (B/X et al) Cave Bear. I imagine the solo adventurer as being a 4th level Fighter (YOU are the Hero).
Hope I've done it justice.