Ceraks

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

Aversion to Fire. If Ceraks takes fire damage, it has disadvantage on attack rolls and ability checks until the end of its next turn.

Legendary Resistance (2/Day). If Ceraks fails a saving throw, it can choose to succeed instead.

Innate Spellcasting. Ceraks' innate spellcasting ability is Intelligence (spell save DC 18). It can innately cast the following spells, requiring no material components:

At will: charm person (as 5th-level spell), detect thoughts, hold person

3/day each: confusion, dimension door, dominate person, fear, hallucinatory terrain, hold monster, hypnotic pattern, telekinesis

Sulfurous Impersonation. As a bonus action, Ceraks can extrude a piece of itself that assumes the appearance of one Medium or smaller creature whose memories it has stolen. This simulacrum appears, feels, and sounds exactly like the creature it impersonates, though it smells faintly of sulfur. Ceraks can impersonate 6 different creatures as a simulacrum that can move up to 120 feet away. For all practical purposes, the simulacrum is Ceraks, meaning Ceraks occupies its space and the simulacrum’s space simultaneously.

Each simulacrum rolls initiative and has 5d12hp (35).

Ceraks has resistance to all damage as long as one simulacrum is out. 

Multiattack. Ceraks makes two pseudopod attacks and uses Eat Memories or three pseudopod attacks.

Pseudopod. Melee Weapon Attack: +7 to hit, reach 10 ft., one target. Hit: 17 (4d6 + 3) bludgeoning damage plus 7 (2d6) psychic damage. Ceraks can attack through their simulacrums. 

Eat Memories. Ceraks targets one creature it can see within 5 feet of it. The target must succeed on a DC 18 Wisdom saving throw or take 44 (8d10) psychic damage and become memory drained until it finishes a short or long rest or until it benefits from the greater restoration or heal spell. Constructs, oozes, plants, and undead succeed on the save automatically.

While memory drained, the target must roll a d4 and subtract the number rolled from any ability check or attack roll it makes. Each time the target is memory drained beyond the first, the die size increases by one: the d4 becomes a d6, the d6 becomes a d8, and so on until the die becomes a d20, at which point the target becomes unconscious for 1 hour. The effect then ends.

When Ceraks causes a target to become memory drained, Ceraks learns all the languages the target knows and gains all its proficiencies, except any saving throw proficiencies.

Ceraks can take 3 legendary actions, choosing from the options below. Only one legendary action option can be used at a time and only at the end of another creature's turn. Ceraks regains spent legendary actions at the start of its turn.

Pseudopod Attack - Ceraks can attack through any of their simulacrum.

Summon or Devour Simulacrum - Summon one simulacrum within 30 ft that has 4d10HP and 16AC taken from Ceraks' health pool. Devour one simulacrum into Ceraks’ main body. Ceraks’ regains health equal to the health of the simulacrum when it was devoured.

Psychic Crush - (Costs 2 Actions) Ceraks targets one creature that it can sense within 60 feet of it. The target must make a DC 10 Intelligence saving throw, taking 10 (3d6) psychic damage on a failed save, or half as much damage on a successful one.