Anti-Magic Elder Oblex

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

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

Corrode Magic. Any magical weapon that hits the arcane slime corrodes. After dealing damage, the weapon takes a permanent and cumulative −1 penalty to damage rolls. If its penalty drops to −5, the weapon is destroyed. Magical ammunition made of metal that hits the ooze is destroyed after dealing damage.

A medium sized ooze can dissolve a small non-artifact magical item no more than 2 feet long in 1d4 rounds and up to 4ft long if it is large sized ooze. Each time the arcane ooze dissolves a magical item it regains 2d8 hitpoints for common, 3d8 uncommon, 4d8 rare, and 5d8 for a very rare item.

Magical Resilience. The arcane slime has resistance to damage and advantage on all saving throws verses magical effects and spells as well as resistance to damage from magical weapon attacks. 

Arcane Armor. The arcane slime is constantly protected by a Mage Armor spell unless it is in an area of anti-magic.

Innate Spellcasting. The oblex’s 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, the 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 oblex can impersonate 3d6 + 4 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 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.

Multiattack. The elder oblex makes two Pseudopod Attacks and uses Eat Memories.

Pseudopod. Melee Weapon Attack: +7 to hit, reach 10 ft., one target. Hit: 17 (4d6 + 3) bludgeoning damage plus 12 (2d6 + 5) psychic damage.

Eat Memories. The oblex 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 an oblex causes a target to become memory drained, the oblex learns all the languages the target knows and gains all its proficiencies, except any saving throw proficiencies.