Armor Class 14 (natural armor)
Hit Points 136 (13d10+65)
Speed 30 ft., fly 20 ft., swim 20 ft.
Saving Throws Str +8, Con +8
Skills Perception +2, Athletics +8
Damage Resistances bludgeoning, piercing, and slashing from nonmagical weapons
Damage Immunities psychic
Condition Immunities charmed, frightened, prone
Senses blindsight 30 ft., darkvision 120 ft., passive Perception 12
Languages understands Abyssal but cannot speak
Challenge 8 (3,900 XP)
Aberrant Form. The Abomination can move through a space as narrow as 1 foot wide without squeezing. It can occupy the same space as another creature.
Psychic Immunity. The Abomination is immune to psychic damage and to any effect that would read its mind or emotions. It has no mind to read.
Nightmare Aura. Any creature that starts its turn within 20 feet of the Abomination must succeed on a DC 15 Wisdom saving throw or become frightened of it until the start of its next turn. On a successful save, the creature is immune to this effect for 24 hours.
Actions
Multiattack. The Abomination makes two attacks: one with its Rending Limb and one with its Maw.
Rending Limb. Melee Weapon Attack: +8 to hit, reach 10 ft., one target. Hit: 18 (3d8+5) slashing damage. On a critical hit, the target must succeed on a DC 16 Constitution saving throw or lose the use of one limb (GM's choice) for 1 minute.
Maw. Melee Weapon Attack: +8 to hit, reach 5 ft., one target. Hit: 14 (2d8+5) piercing damage plus 14 (4d6) psychic damage.
Void Scream (Recharge 5-6). The Abomination emits a sound from outside normal reality in a 30-foot cone. Each creature in that area must succeed on a DC 15 Constitution saving throw or take 35 (10d6) psychic damage and be stunned until the end of its next turn. On a success, the creature takes half damage and isn't stunned.
Reactions
Absorb Impact. When the Abomination takes bludgeoning, piercing, or slashing damage from a nonmagical weapon, it can use its reaction to halve the damage (it is resistant but explicitly halves rather than just resisting).
Lore
Xaruun is the goddess of monsters and nightmares, Queen of the darkest places in all the worlds -- and her followers are rewarded with exactly what they sought: something uncontrollable. A Xaruun-Touched Abomination is what emerges when a summoning ritual to Xaruun succeeds beyond the summoner's ability to manage. The creature that answers is assembled from the nightmare-fragments Xaruun keeps in her domain: too many limbs, wrongly jointed; a mouth that opens sideways; eyes that do not track together; a mass of dark matter that follows it like a shadow and does not behave like a shadow should. The summoner is usually the first thing it kills. Most Xaruun-priest martyrs are not martyrs by choice.
Habitat
ruins, summoning sites, locations where the barrier between planes has been weakened