Legendary Resistance (4/Day). If Zathyxis fails a saving throw, it can choose to succeed instead.

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

Fiendish Rebirth. Dexterity Saving Throw: DC 23, when Zathyxis is reduced to 0 HP for the first time, he explodes in a burst of apocalyptic fire, dealing 30 fire and 30 necrotic damage in a 60 ft. radius. Then, he reforms in a pillar of black flame at the start of the next round with half HP (180) and loses access to spellcasting, but all his attacks deal additional 10 necrotic damage. Success: Half damage only

Multiattack. Zathyxis makes two attacks, using Unholy Glaive or Voidfire Ray in any combination.

Unholy Glaive. Melee Attack Roll: +17, reach 10 ft. Hit: 36 (6d8 + 10) Force damage, and the target has the Stunned condition until the start of Zathyxis' next turn. The target can choose not to be Stunned, in which case it takes an extra 21 Force damage that bypasses Resistance or Immunity.

Voidfire Ray. Ranged Attack Roll: +15, range 600 ft. Hit: 42 (8d8 + 8) Necrotic or Fire damage (Zathyxis' choice).

Spellcasting. Zathyxis casts one of the following spells, requiring no Material components and using Charisma as the spellcasting ability (spell save DC 23, +15 to hit):

At Will: Hellish Rebuke, Fear, Detect Magic, Dispel Magic
1/Day Each: Fire Storm, Dispel Evil and Good, Plane Shift

Legendary Action Uses: 3. Immediately after another creature’s turn, Zathyxis can expend a use to take one of the following actions. Zathyxis regains all expended uses at the start of each of its turns.

Bolster. Zathyxis gains 10 Temporary Hit Points, and Zathyxis and each ally within 30 feet of it gain Advantage on D20 Tests until the end of Zathyxis' next turn. Zathyxis can’t take this action again until the start of its next turn.

Shockwave of Ruin. Constitution Saving Throw: DC 23, each creature in a 30-foot Emanation originating from Zathyxis. Failure: 26 (8d8) Necrotic damage, and the target has the Prone condition. Success: Half damage only. Failure or Success: Zathyxis can’t take this action again until the start of its next turn.

Smite. Zathyxis makes one Voidfire Ray attack.