Aspiring Abomination

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

Mutated Mind. Any psychic damage dealt to the Abomination is turned back onto the creature who dealt the damage. In addition, the Abomination is immune to any ability that would prevent it from not being able to use its action economy.

Headlamp. The Abomination has a bioluminescent headlamp that dangles from its head. This lamp produces bright light in a 15 foot radius, and dim light in an additional 15 foot radius. Whenever a creature enters this light for the first time on its turn, or starts its turn there, they must succeed on a DC 15 Wisdom saving throw. On a failed save, the creature is stunned until the end of the Abomination's next turn, and must use whatever movement it has to approach the Abomination, attempting to end their turn within 5 feet of the Abomination.

Innate Spellcasting (Psionics). The mind flayer is a 14th-level spellcaster. Its innate spellcasting ability is Intelligence (spell save DC 22; +14 to hit with spell attacks). It can innately cast the following spells, requiring no components:

At will: guidance, mage hand, vicious mockery, true strike

1st level (4 slots): charm person, command, comprehend languages, sanctuary

2nd level (3 slots): crown of madness, phantasmal force, see invisibility

3rd level (3 slots): clairvoyance, fear, meld into stone, gravity fissure

4th level (3 slots): confusion, stone shape, greater invisibility, [spell]gravity sinkhole/spell]

5th level (2 slots): scrying, telekinesis, synaptic static

6th level (1 slot): gravity fissure

7th level (1 slot): power word pain

Multiattack. The Abomination makes three attacks: one with its bite, two with its stomp. The Abomination can substitute its bite attack with its swallow attack.

Toothy Maw. Melee Weapon Attack: +13 to hit, reach 5 ft., one target. Hit: 34 (4d12 + 8) piercing damage, plus 11 (2d10) psychic damage. If the target is a creature, it must succeed on a DC 22 Strength saving throw or be grappled.

Slam. Melee Weapon Attack: +13 to hit, reach 10 ft., one target. Hit: 34 (4d12 + 8) piercing damage. The Abomination can forgo the damage and grapple the target (escape DC 19). While grappled the target is restrained and the Abomination can use the target as half cover. It can have 

Swallow. The Abomination makes one bite attack against a Large or smaller creature it is grappling. If the attack hits, that creature takes the bite's damage and is swallowed, and the grapple ends. While swallowed, the creature is blinded and restrained, it has total cover against attacks and other effects outside the Abomination, and it takes 18 (6d6) acid damage at the start of each of the Abomination's turns.

If the Abomination takes 30 damage or more on a single turn from a creature inside it, the Abomination must succeed on a DC 15 Constitution saving throw at the end of that turn or regurgitate all swallowed creatures, which fall prone in a space within 10 feet of the Abomination. If the Abomination dies, a swallowed creature is no longer restrained by it and can escape from the corpse using 15 feet of movement, exiting prone.

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

Reflexive Magic (Costs 2 Actions). The Abomination can cast a spell of 3rd level or lower.

Stomp. The Abomination makes a slam attack.

Swallow (Costs 2 Actions). The Abomination can attempt to swallow one creature it is holding.