Delayed Destruction (Mythic Trait; Recharges after a Short or Long Rest). If Zelth is reduced below 200 hit points, at the start of his turn he billows dark mist causing his Armor Class to become 26 he regains 250 hit points, and can teleport immediately without using any actions. These regained hitpoints cannot be stopped or effected by spells like chill touch or other similar effects short of a Wish spell.

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

Spellcasting. Zelth is a 9th-level spellcaster. Its spellcasting ability is Charisma (spell save DC 21, +18 to hit with spell attacks). Zelth has following spells prepared:

Cantrips (at will): Toll the Dead, Faerie Fire, Bestow Curse

1st level (8 slots): Dissonant Whispers, Thunderwave, Hex, Hunter's Mark, Magnify Gravity, Witch Bolt

2nd level (5 slots): Blindness/Deafness, Cloud of Daggers, Ray of Enfeeblement, Silence

3rd level (5 slots): Animate dead, Counterspell, Dispel Magic, Fireball, Slow

4th level (5 slots): Blight, Gravity Sinkhole, Sickening Radiance

5th level (5 slots): Cloudkill, Contagion, Destructive Wave, Dominate Person, Immolation

6th level (3 slot): Disintegrate, Sunbeam, Harm

7th level (3 slot): Finger of Death, Tether Essence

8th level (2 slot): Sunburst, Reality Break, Dark Star

9th level (2 slot): Mass Polymorph, Storm of Vengeance

Drain Aspect. When a creature is hit with a Draining tentacle it must roll 1d12 and be afflicted by the corresponding number. Nothing short of a wish spell or other powerful magics can undo these effects.

Multiattack. Zelth can use its Eyes of entropy. It then makes three attacks one with its Drain attack and two with its Tentacle attack.

Tentacle. Melee Weapon Attack: +19 to hit, reach 15 ft., 2 target. Hit: 23 (2d12 + 12) bludgeoning damage. 

Eye of Entropy.  Zelth chooses one creature to be marked. A marked creature has disadvantage on any save using one ability of Zelth's choice. 

Drain. Melee Weapon Attack: +13 to hit, reach 20 ft., 1 target. Hit:  Drain Aspect.

Cataclysmic Cascade (Recharge 5–6). Reality bends and cracks in a 60ft cone. All creatures in the cone must make a Con save DC 18. On a failed save the creature takes 8d10 force or half as much on a successful save.

Screams of the Undone (Recharge 6). A mouth opens on Zelth's body releasing the screams of those unmade. All creatures within 400ft that can hear it must make a Wisdom save DC 18. On a failed save the creature is stunned until the beginning of Zelth's next turn, and take 3d12 psychic damage. On a successful save the creature is not stunned and takes half damage. If a creature has failed against this effect they have advantage on all future saves against this effect.

Teleport. As a bonus action, Zelth can teleport up to 80 feet to an unoccupied space it can see.

Sap. When hit with a melee weapon attack or melee spell attack Zelth can make a Drain attack at disadvantage with only +5 to hit.

Zelth 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. Zelth regains spent legendary actions at the start of its turn.

Slam. Zelth makes one Tentacle attack.

Drain (Costs 2 Actions). Zelth makes one Drain attack.

If Zelth's Chaos Unbound trait has activated in the last hour, it can use the options below as legendary actions.

Undone (Costs 1 Action). Zelth makes one Cataclysmic Cascade or one Screams of the Undone attack.

Reality Warp (Costs 3 Actions). Zelth can create a tear in reality that shapes the fabric of the universe. Treat this as if Zelth cast the Wish spell.