Legendary Resistance (2/Day). If the nightcrawler fails a saving throw, he can choose to succeed instead.

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

Gas Cube. The cube takes up its entire space. Other creatures can enter the space, but a creature that does so is subjected to the cube's Engulf and has disadvantage on the saving throw.

Creatures inside the cube can be seen but have total cover.

A creature within 5 feet of the cube can take an action to pull a creature or object out of the cube. Doing so requires a successful DC 18 Strength check, and the creature making the attempt takes 4d6 psychic damage.

The cube can hold only one Huge creature or up to four Large or smaller creatures inside it at a time.

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

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 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 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.

Engulf. The cube moves up to its speed. While doing so, it can enter Large or smaller creatures' spaces. Whenever the cube enters a creature's space, the creature must make a DC 18 Dexterity saving throw.

On a successful save, the creature can choose to be pushed 5 feet back or to the side of the cube. A creature that chooses not to be pushed suffers the consequences of a failed saving throw.

On a failed save, the cube enters the creature's space, and the creature takes 3d6 psychic damage and is engulfed. The engulfed creature can't breathe, is restrained, and takes 7d6 psychic damage at the start of each of the cube's turns. When the cube moves, the engulfed creature moves with it.

An engulfed creature can try to escape by taking an action to make a DC 18 Charisma check. On a success, the creature escapes and enters a space of its choice within 5 feet of the cube.