Avatar of Kyuss

Discorporation. If Kyuss is reduced to 0 hit points he falls into a pile
of giant vermin (worms, maggots, etc.) that immediately attempt to
escape by slithering through holes or burrowing into the earth and
his equipment is banished to a demiplane only Kyuss can access.
If at least one vermin escapes, of approximately 2 million,
Kyuss can reform his body, at full strength, in (1d100) days. This
time may be drastically shortened depending on the number of vermin that escape, but it is never less than 100 years.

Innate Spellcasting. Kyuss’ spell casting ability is Intelligence (spell
save DC 23, +15 to hit with attack spells). Kyuss can innately cast
the following spells, requiring no material components:
At-will: animal shapes, animate dead, protection from good and evil
3/day each: conjure animals, create undead, harm, insect plague
1/day each: abdi-dalzim’s horrid wilting, contagion, finger of death

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

Swarm Form. Kyuss can move through a space as narrow as one feet wide without squeezing.

Withering Touch. A creature that Kyuss hits with a melee attack, touches Kyuss, or hit him with a melee attack within 10 feet of him takes (6d6) necrotic damage and Kyuss regains an equal amount of hit points.

Multiattack. Kyuss can use his Frightful Presence and then make two mace or slam attacks.

Mace. Melee Weapon Attack: +17 to hit, reach 15 ft., one target. Hit: 41 (4d8+19) bludgeoning damage, plus 36 (4d8) necrotic damage and the target must make a DC23 Strength saving throw or be knocked prone or pushed 20 feet, Kyuss' choice.

Slam. Melee Weapon Attack: +17 to hit, reach 10 ft., one target. Hit: 35 (4d6+18) bludgeoning damage, plus 36 (4d8) necrotic damage. See also withering touch.

Frightful Presence. Each creature of Kyuss' choice within 300 feet of him and aware of him must succeed on a DC23 Wisdom saving throw or become frightened for 1 minute. A creature can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success. If a creature's saving throw is successful or the effect ends for it, the creature is immune to Kyuss' Frightful Presence for the next 24 hours.

Engulf. Kyuss can move up to his speed and enter the space of one Huge or multiple smaller creatures, swallowing them in a mass of vermin. All targets in the area must make a DC23 Constitution saving throw, taking 55 (10d10) piercing damage plus (4d8) necrotic damage and be grappled (escape DC 23) on a failed save and half as much damage on a successful one. Grappled creatures are also restrained and must escape the grapple and move outside of Kyuss' space or take the same damage at the end of each of their turns. 
An engulfed creature that succeeds on escaping the grapple must move outside of Kyuss' space or be grappled again at the end of its turn. Kyuss' space is considered difficult terrain.

Worm Burst (Recharge 6). Kyuss selects a point he can see within 500 feet of him, conjuring necrotic worms. Each creature within a 20-foot radius of the point must make a DC 23 Dexterity saving throw, taking 55 (10d10) piercing damage plus 36 (8d8) necrotic damage and gaining a level of exhaustion on a failed save and half as much damage on a successful one.

Quick Casting. Kyuss casts an at-will spell.

Reaction Name. Enter the description for your action.

Attack. Kyuss makes a mace or slam attack.

Quick Casting. Kyuss can cast an at-will spell.

Move. Kyuss can move up to half his speed.

Spellcasting (Costs 2 Actions). Kyuss casts a spell.

Teleport (Costs 2 Actions). Kyuss teleports to an unoccupied space he can see within 180 feet of him.

Worm Burst (Costs 3 Actions). Kyuss uses a worm burst attack.