Energy Eater. When the abilisk takes lightning damage, instead of losing hp, it gains hp equal to the amount of lightning damage it took. 

Legendary Resistance (4/day). When the abilisk fails a saving throw, it can choose to succeed instead. 

Multiattack. The abilisk makes 4 tentacle attacks and one bite attack.

Tentacle. Melee Weapon Attack: +16 to hit, reach 20 ft., 1 target. Hit: 17 (2d10 + 5) bludgeoning damage and the target is grappled. While grappled in this way, the target is also restrained.

Bite. Melee Weapon Attack: +16 to hit, reach 5 ft., 1 target. Hit: 22 (4d10) piercing damage. In addition, if the target bitten is large or smaller, the abilisk can swallow it. While swallowed, a creature takes 5d10 acid damage at the end of each of the abilisk's turns. If the abilisk takes 50 or more damage in a single turn from the creature in its belly, it must succeed on a DC 22 constitution save or spit the creature out prone. If the abilisk dies, the target can use 20 feet of movement to exit the abilisk prone.

Multidimensional Breath. (Recharge 5-6) The abilisk breathes out a blast suffused with the energy of the multiverse in a 30 foot cone. Every creature in that area must make a dexterity saving throw, DC 19, or take 10d10 radiant damage. A creature who succeeds takes half damage.

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

Move (Costs 1 Actions). The abilisk moves up to half of its speed. 

Tentacle (Costs 2 Actions). The abilisk makes one tentacle attack.

Thunderous Bellow (Costs 3 Actions) Every creature within 10 feet of the abilisk must succeed on a DC 19 constitution saving throw or take 2d8 thunder damage and be knocked prone.