Cryptsoul

Amorphous. The Cryptsoul can move through a space as narrow as 1 inch wide without squeezing.

Shadow Stealth. While in dim light or darkness, the Cryptsoul can take the Hide action as a bonus action.

Sunlight Weakness. While in sunlight, the Cryptsoul has disadvantage on attack rolls, ability checks, and saving throws.

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

Innate Spellcasting (Psionics). The Cryptsoulr'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)

Life draining tentacles . Melee Weapon Attack: +7 to hit, reach 5 ft., one target. Hit: 15 (2d10 + 4) psychic damage. If the target is Medium or smaller, it is grappled (escape DC 15) and must succeed on a DC 15 Intelligence saving throw or be stunned until this grapple ends and the target's Strength score is reduced by 1d4. The target dies if this reduces its Strength to 0. Otherwise, the reduction lasts until the target finishes a short or long rest.

Extract Brain. Melee Weapon Attack: +9 to hit, reach 5 ft., one incapacitated humanoid grappled by the Cryptsoul. Hit: The target takes 55 (10d10) necrotic damage. If this damage reduces the target to 0 hit points, the mind flayer kills the target by extracting and devouring its brain.

If a non-evil humanoid dies from this attack, a new Cryptsoul rises from the corpse 1d4 hours later.

Mind Blast (Recharge 5-6). The Cryptsoul 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 22 (4d8 + 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. This psychic blast can be absolutely devastating to a party in combat. It's important to note that a stunned victim automatically fails strength and dexterity saving throws.