Everchanging Appearance. Roll a d10 at the beginning of combat. Based off of the outcome, this creature's appearance will shift. 

1. (1-2) Mindflayer. The Abomination may make a Psychic attack on a creature rather than a Multiattack. The target must succeed on a DC 14 Wisdom saving throw or take 2d4 Psychic damage.
2. (3-4) Gibbering Mouther. In a 10 ft. radius around the Abomination, the ground is considered difficult terrain.
3. (5-6) Nothic. The Abomination targets one creature within 30 ft., that target must make a DC 14 Wisdom saving throw, or take 3 (1d6) necrotic damage.
4. (7-8) Oblex. The Abomination is considered amorphous and can fit through any space as narrow as 1 inch. 
5. (9-10) Beholder. The abomination grows two eyestalks, Enervation Ray and Paralyzing Ray. Target must succeed on a DC 14 Constitution save for either ray or take 2d6 necrotic damage or be paralyzed for a minute respectively.

Multiattack. The Abomination makes two attacks, one with its Slam attack and another with its Rake attack.

Slam. Melee weapon attack. +6 to hit, reach 5ft., one target. Hit: 7 (1d8+3) bludgeoning damage. The target must make a Strength saving throw (DC 13) or be pushed back 10 ft. and knocked prone.

Rake. Melee weapon attack, +6 to hit, reach 10ft., one target. Hit: 9 (2d6+3) slashing damage.