Legendary Resistance (8/day). If Zyxilum fails a saiving throw, he can choose to succeed instead.

Primordial Form. Any spells that would directly affect Zyxilum's form, such as polymorph, automatically fail.

Bad Luck. The Wish spell cannot directly affect Zyxilum.

Ancient Constitution. At the start of each of his turns, Zyxilum regenerates 200 hit-points.

Teraeon Existence. As a being who is over a trillion years old, Zyxilum haas advantage on all ability checks.

Magical Existence. All of Zyxilum's attacks deal magical damage.

Ancient Amalgamation. As a being composed of three individuals, Zyxilum has advantage on wisdom and intelligence saving throws, but disadvantage on charisma saving throws.

 Multiattack. Zyxilum makes five attacks: four with his tentacles and one with his Gaze of the Ancients. 

Tentacle. Melee Weapon Attack: +19 to hit, reach 20 ft., 1 target. Hit: 32 (4d10 + 10) force damage and 22 (4d10) acid damage, the target must also make a strength saving throw (DC 30), on a failed save, the target is grappled. A creature grappled in this way takes an additional 17 (3d10) acid damage at the start of each of its turns, and it can attempt to escape at the end of its turns, re-rolling the strength save and ending the effect on a success.

Gaze of the Ancients.  Roll a d4, this magical attack affects a number of creatures equal to the number rolled. Each effected creature must make a DC 34 constitution saving throw. On a failure and effected creature takes 12d12 radiant damage and is stunned until the end of its next turn. On a success, the creature takes half damage and is not stunned.

Anti-Probability Field (3/day). Zyxilum can impose disadvantage on all creatures of his choice until the end of his next turn.

Zyxilum has five legendary actions available per round of combat, and all expended actions are regained at the start of his turn. He can only take legendary actions after another creature takes its turn.

Tentacle (1 action). Zyxilum makes a tentacle attack.

Unseen Abomination (2 actions, 1/day). Zyxilum and his allies turn invisible, and remain that way until they take damage.

Eyes of the Old Ones (4 actions). Zyxilum and his allies have advantage on all rolls for two full rounds of combat.

If Zyxilum drops to zero hit-points, he returns to 800 hit-points, regains his legendary resistances and one use of his Anti-Probability Field. He also unlocks his mythical actions, which he can take along with his legendary actions. He has three mythical actions that he can take on initiative counts 30, 20, and 10. He can only use each action once per round. He regains all used actions at the top of initiative.

Celestial Pulse. Every creature hostile to Zyxilum takes 75 points of radiant damage.

Your Soul is Forfeit. One creature of Zyxilum's choice turns on their allies for one round. On the creature's next turn, it takes actions as normal, believing it is attacking its enemies. All attacks and spells cast by the creature are against is allies for the duration of its turn. Once its turn is complete, it realizes what it did.

You Cannot Pass. All hostile creatures are immobile until the end of their next turn.