Magic Resistance. Abyssalix has Advantage on saving throws against spells and other magical effects.

Dooming Presence. Any creature that starts its turn within 30 ft. of Abyssalix must succeed on a DC 23 CON saving throw or take 23 (6d6) psychic damage. 

Unusual Nature. Abyssalix doesn't require air, food, drink, or sleep.

Legendary Resistance (3/Day) If Abyssalix fails a saving throw, it can choose to succeed instead.

Multiattack. Abyssalix makes four Bite attacks, two Sting attacks, and one Ghastly Breath attack.

Bite. Melee Attack Roll: +6, reach 5 ft. Hit: 21 (6d6) Piercing damage.

Sting. Constitution Saving Throw: DC 21, one creature that Abyssalix can see within 5 feet. Failure: 16 (4d6) Poison damage, and the target has the Poisoned condition for 1 hour. Failure by 5 or More: The Poisoned target also has the Unconscious condition until it takes damage or another creature takes an action to shake it awake.

Ghastly Breath. Ranged Attack Roll: +6, reach 15 ft. Hit: 23 (6d6 + 2) Psychic damage.

Teleport. Abyssalix teleports, along with any equipment he is wearing or carrying, to an unoccupied space within 60 ft. of him.

Abyssal Revenge. When Abyssalix gets hit by a Melee or Ranged attack, he may use a reaction to use a Ghastly Breath attack on the attacker.

Abyssal Sanctuary. When Abyssalix gets hit by a Melee or Ranged attack, it may use a reaction to teleport into a pocket dimension for up to one minute. While in this dimension, Abyssalix cannot be located, tracked, or followed in any way. Abyssalix also regains 1d6 HP while in this dimension. When Abyssal Sanctuary ends, Abyssalix can choose to appear in any empty space within 30 ft. of where he was when he entered Abyssal Sanctuary.

Abyssalix 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. Abyssalix regains spent legendary actions at the start of his turn.

Attack. Abyssalix makes one Ghastly Breath attack.

Teleport. Abyssalix uses Teleport.

Hide. Abyssalix becomes invisible for a period of one turn.