Arcane Oblex

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

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

Sulfurous Impersonation. As a bonus action, the arcane oblex 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. The arcane oblex can impersonate 2d6 + 1 different creatures, each one tethered to its body by a strand of slime that can extend up to 120 feet away. For all practical purposes, the simulacrum is the arcane oblex, meaning that the arcane oblex 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 the arcane oblex’s main body and the simulacrum. The simulacrum disappears if the tether is severed.

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

3/day each:  charm person (as 5th-level spell), color spray, detect thoughts, hold person (as 3rd-level spell)

Simulacrum Spellcasting.  Stealing the memories of magic users allows the oblex to memorize wizard spells and cast them through their simulacrum. 

The arcane oblex is a 3rd-level spellcaster. Its spellcasting ability is intelligence (spell save DC 15, +7 to hit with spell attacks). The arcane oblex has following wizard spells prepared:
Cantrips (at will): prestidigitation, ray of frost, light 
1st level (4 slots): ray of sickness, grease, shield

2nd level (2 slots): phantasmal force, darkness

3rd level (1 slot): fear

Multiattack.  The arcane oblex makes two pseudopod attacks and uses Eat Memories.

Pseudopod. Melee Weapon Attack: +7 to hit, reach 10 ft., one target. Hit: 10 (2d6 + 3) bludgeoning damage plus 7 (2d6) psychic damage.

Eat Memories. The arcane oblex targets one creature it can see within 5 feet of it. The target must succeed on a DC 15 Wisdom saving throw or take 18 (4d8) 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 an arcane oblex causes a target to become memory drained, the arcane oblex learns all the languages the target knows and gains all its proficiencies, except any saving throw proficiencies.