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

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

Innate Spellcasting. Yisneh's innate spellcasting ability is Intelligence (spell save DC 19). It can innately cast the following spells, requiring no material components:

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

5/day each: abi-dalzim’s horrid wilting, confusion, crown of madness, dimension door, dominate person (as a 7th level spell), fear, hallucinatory terrain, hold monster, hypnotic pattern, telekinesis

Sulphurous Impersonation. As a bonus action, Yisneh 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. Yisneh can impersonate up to 30 different creatures, each one tethered to its body by a strand of transparent slime that can extend up to 200 feet away. For all practical purposes, the simulacrum is Yisneh, meaning Yisneh occupies its space and the simulacrum's space simultaneously. The slimy tether is immune to damage, but it is severed if there is no opening at least 1 inch wide between Yisneh's main body and the simulacrum. The simulacrum disappears if the tether is severed.

Legendary Resistance (3/day): If Yisneh fails a saving throw, it can choose to succeed instead.

Multiattack. Yisneh makes two pseudopod attacks and uses Eat Memories.

Pseudopod. Melee Weapon Attack: +9 to hit, reach 15 ft., one target. Hit: 25 (6d6 + 4) bludgeoning damage plus 11 (3d6) psychic damage.

Eat Memories. Yisneh targets one creature it can see within 10 feet of it. The target must succeed on a DC 19 Wisdom saving throw or take 70 (14d10) psychic damage and become memory drained until it finishes a long or short 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 d6 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 d6 becomes a d8, the d8 becomes a d10, and so on until the die becomes a d20, at which point the target becomes unconscious for 1 hour. The effect then ends.

     When Yisneh causes a target to become memory drained, Yisneh learns all the languages the target knows, and gains all its proficiencies.

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

Offspring. If Yisneh has consumed memories within the last 24 hours, a adult oblex splits off from it. This action cannot be taken again until Yisneh consumes the memories of another creature.

Mesmerize. A creature must succeed on a DC 18 wisdom saving throw or become entranced by Yisneh's haunting form, becoming paralyzed until the end of Yisneh's next turn.

Absorb (Last Resort: can't be used unless below 59 hp). Yisneh uses Eat Memories. If it succeeds, Yisneh envelopes the target, dissolving its body and consuming its memories.