Greater Regeneration. Zargon regains 30 hit points at the start of his turn. If he takes radiant damage, he only regains 15 hit points at the start of his next turn. Zargon dies only if he starts his turn with 0 hit points and has taken at least 30 radiant damage since the start of his last turn.

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

Magic Resistance. Zargon has advantage on saving throws against spells and other magical effects.

Rejuvenation. If Zargon is slain, his body disintegrates into goo, except for his horn. 1d100 days after Zargon is slain, his body completely reforms around the horn, and Zargon regains all of hit points.

Zargon's horn has immunity to all damage and can't be destroyed by conventional or magical means. However, if a wish spell is cast on Zargon's horn while he is dead, it takes an additional 1d100 years for him to reform.

Multiattack. Zargon makes one Bite attack, one Horn attack and three Tentacle Claw attacks. If all three Tentacle Claw attacks hit the same Large or smaller creature, that creature is also grappled (escape DC 27) and is restrained until the grapple ends. Zargon can have up to six creatures grappled in this way at a time.

Bite. Melee Weapon Attack: +19 to hit, reach 5 ft., one target. Hit: 29 (3d12 + 10) magical piercing damage plus 19 (3d12) poison damage, and the target must succeed on a DC 26 Constitution saving throw or be poisoned. A creature poisoned in this way can only be cured by restorative magic such as lesser restoration.

Horn. Melee Weapon Attack: +19 to hit, reach 10 ft., one target. Hit: 26 (3d10 + 10) magical piercing damage.

Tentacle Claw. Melee Weapon Attack: +19 to hit, reach 20 ft., one target. Hit: 20 (3d6 + 10) magical slashing damage plus 13 (2d12) acid damage.

Hellish Nightmare. Zargon targets one creature that he can see within 90 feet of him. The target must succeed on a DC 26 Charisma saving throw or be teleported to a hellish demiplane for 1 minute. A creature can repeat the saving throw at the start of each of its turns, returning to the closest available spot near where it left.

At the end of each of the creature's turns within the demiplane, it must succeed on a DC 26 Wisdom saving throw or take 16 (3d10) psychic damage. If a humanoid dies while within the demiplane, its body and soul are transformed into a nupperibo and is returned to the closest available spot near where it left. Zargon can only have one creature within the demiplane at a time.

Spellcasting. Zargon casts one of the following spells, requiring no material components and using Charisma as the spellcasting ability (spell save DC 26):

At will: detect magic, dispel magic, fear, locate object, suggestion

3/day: disintegrate, hold monster, locate creature, major image

1/day: weird

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

Crackling Beam. Zargon releases a beam of energy from his eye at a creature that he can see within 120 feet of him. The creature must make a DC 26 Dexterity saving throw, taking 66 (12d10) lightning damage on a failed save, or half as much damage on a successful one.

Savage. Zargon makes one Bite attack and one Horn attack.

Eviscerate (Costs 2 Actions). Each creature grappled by Zargon must succeed on a DC 26 Constitution saving throw or take 55 (10d10) magical slashing damage.