Ancient Obelex

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

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

Spellcasting: The ancient oblex's innate spellcasting ability is Intelligence (spell save DC 22, +15 to hit with spell attacks). It can innately cast the following spells, requiring no material component

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

3/day each: confusion, dimensions door, dominates person, fear, hallucinatory terrain, hold monster, hypnotic pattern, telekinesis, slow, silence.

Sulfurous Impersonation. As a bonus action, the oblex can extrude a piece of itself that assumes the appearance of one large 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 ancient oblex can impersonate (3d6 + 1) a lot different creatures, each one tethered to its body by a strand of slime that can extend up to 300 feet away. For all practical purposes, the simulacrum is the oblex, meaning the 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 oblex’s main body and the simulacrum. The simulacrum disappears if the tether is severed.

Corrosive Form. A creature that touches the ancient obelex or hits it with a melee attack while within 10 feet of it takes 4 (3d8) acid damage. Any nonmagical weapon made of metal or wood that hits the penalty drops to −5, the weapon is destroyed. Nonmagical ammunition made of metal or wood that hits the ancient obelex is destroyed after dealing damage.The ancient obelex can eat through 2-inch-thick, nonmagical wood or metal in 1 round.

Ooze nature. The ancient oblex doesn’t require sleep 

Multiattack. The ancient obelex makes three pseudopod attacks and uses Eat Memories.

 Pseudopod. Melee Weapon Attack: +12 to hit, reach 20 ft., one target. Hit: 17 (6d6 + 3) bludgeoning damage plus 9 (3d6) psychic damage, plus 4 (3d8) acid damage.

 Eat Memories. The ancient Obelex targets one creature it can see within 20 feet of it. The target must succeed on a DC 22 Wisdom saving throw or take 44 (12d10) 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 obelex causes a target to become memory drained, the obelex takes learns all the languages the target knows and gains all its proficiencies, except any saving throw proficiencies.

The ancient obelex 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. The ancient obelex regains spent legendary actions at the start of its turn.

Pseudopod(1 charge). The ancient obelex can make one pseudopod attack.

Spell(2 charges). The ancient obelex can cast one spell of it's choice.

Eat Memories.(3 charges). the ancient obelex can use it's eat memories ability.