Shapechanger. The Illithimimic can use its action to polymorph into an object or back into its true, amorphous form. Its statistics are the same in each form. Any equipment it is wearing or carrying isn't transformed. It reverts to its true form if it dies.

Adhesive (Object Form Only). The Illithimimic adheres to anything that touches it. A Huge or smaller creature adhered to the Illithimimic is also grappled by it (escape DC 14). Ability checks made to escape this grapple have disadvantage. Additionally, any creature that is adhered to the ilithimimic must succeed on a DC 15 Intelligence saving throw or be stunned for 1 minute. A creature can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success.

False Appearance (Object Form Only). While the Illithimimic remains motionless, it is indistinguishable from an ordinary object.

Grappler. The Ilithimimic has advantage on attack rolls against any creature grappled by it.

Magic Resistance. The Ilithimimic has advantage on saving throws against spells and other magical effects.

Innate Spellcasting (Psionics). The Ilithimimic’s innate spellcasting ability is Intelligence (spell save DC 15). It can innately cast the following spells, requiring no components:

At will: detect thoughts, levitate

1/day each: dominate monster, plane shift (self only)

Pseudopod. Melee Weapon Attack: +6 to hit, reach 5 ft., one target. Hit: 12 (2d8 + 3) psychic damage. If the Illithimimic is in object form, the target is subjected to its Adhesive trait.

Extract Brain. Melee Weapon Attack: +7 to hit, reach 5 ft., one incapacitated humanoid grappled by the Illithimimic. Hit: The target takes 36(8d8) piercing damage. If this damage reduces the target to 0 hit points, the Illithimimic kills the target by extracting and devouring its brain.

Mind Blast (Recharge 5–6). The Illithimimic magically emits psychic energy in a 60-foot cone. Each creature in that area must succeed on a DC 15 Intelligence saving throw or take 18(4d6 + 4) psychic damage and be stunned for 1 minute. A creature can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success.