Ascended Oblex

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

Beyond Ascension (Mythic Trait; Recharges after a Short or Long Rest). If the oblex is reduced to 0 hit points, it doesn’t die or fall unconscious. Instead, it regains 200 hit points. In addition, its Magic Resistance trait activates.

Innate Spellcasting (Psionics). The oblex’s innate spellcasting ability is Intelligence (spell save DC 16). It can innately cast the following spells, requiring no material components:

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

1/day each: power word pain, true seeing

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

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

Magic Resistance (Mythic Trait). The ascended oblex has advantage on saving throws against spells and other magical effects, as long as its Beyond Ascension trait is activated.

Shielded Mind. The ascended oblex is immune to scrying and to any effect that would sense its emotions, read its thoughts, or detect its location.

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 3d8 + 1 different creatures, each one tethered to its body by a strand of slime that can extend up to 240 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 one pseudopod attacks and uses Eat Memories.

Pseudopod. Melee Weapon Attack: +7 to hit, reach 15 ft., one target. Hit: 17 (4d6 + 3) bludgeoning damage plus 7 (2d6) psychic damage.

Eat Memories. The oblex targets one creature it can see within 5 feet of it. The target must succeed on a DC 16 Wisdom saving throw or take 21 (6d6) 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.

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

Alert. The ascended oblex regains its Reaction.
Move. The ascended oblex moves up to its speed.
Sulfurous Impersonation. The ascended oblex uses its Sulfurous Impersonation.

If the oblex's mythic trait is active, it can use the options below as legendary actions for 1 hour after activating Beyond Ascension.

Command. The oblex casts Command.

Eat Memories (Costs 2 Actions). The oblex uses Eat Memories.

Psychic Blast (Costs 3 Actions). The oblex deals 35 (10d6) psychic damage to all creatures currently charmed by it. Those creatures are no longer charmed by the oblex, and are immune to being charmed by the oblex for 24 hours.