Kyuss is a 20th-level spell-caster. His spell-casting ability is Intelligence (spell save DC 20, +10 to hit with spell attacks). Kyuss has the following Wizard spells prepared:

Cantrips (at will): Chill touch, Toll the dead, infestation

1st level (6 slots): Ray of sickness, False life

2nd level (5 slots): Blindness/Deafness, Wither and Bloom

3rd level (5 slots): Animate dead, Bestow curse (requires C if a slot under 5th is used), spirit shroud (BA)(C), summon undead (putrid form only)(C), Summon lesser demon(C) Summon shadows (C)

4th level (4 slots): Summon greater demon (C)

5th level (4 slots): Dance Macabre(they become spawn of Kyuss instead of skeletons or zombies)(C), Negative energy flood(they rise as spawns of Kyuss instead of zombies)

6th level (4 slots): Circle of death, Create undead

7th level (3 slots): Finger of death (they rise as a spawn of Kyuss instead of a zombie), Teleport

8th level (2 slots): Dominate monster(C), feeble mind, Abi-Dalxims horrid wilting

9th level (1 slots): Meteor swarm, invulnerability (C)

Anathematic Secrecy. Any attempt to use divination magic that involves Kyuss will fail unless he wants it to succeed.

Eldritch Mind. Any attempts to dominate Kyuss, read his mind, influence his behaviour, or detect if he is lying will fail unless he wants them to succeed. Masterful Mage. Kyuss has advantage on concentration saving throws.

Magical Resistance. Kyuss has advantage against all spell affects.

Magic Weapons. Weapon attacks made by Kyuss are considered magical for the purposes of overcoming resistances.

Tiny Bodies. Kyuss can fit through any sized hole larger than an inch.

Old Skin. Kyuss can hide his worm form with magic and by doing so make himself appear to look how he did before he became The Worm That Walks, whilst in this state he both looks and feels like an ordinary human. He is not affected by the Ever Growing Form or Frightful Presence (see below) abilities whilst in this sate and he can swap between his human and worm state as an action however he prefers the worm form.

Frightful Presence. Each creature when they first see Kyuss (and again whenever he grows a size category) must succeed on a DC 20 wisdom save or become frightened for the next minute.

Ever Growing Form. As long as he has at least 1 hp Kyuss regains 18 (4d8) hp at the start of each of his turns, he also regains this health whenever he kills a creature. Kyuss can also grow or shrink 1 size category whenever he wishes, doing this takes an entire turn and means he cannot use any other actions or legendary actions until the start of his next turn.

Monstrous Body. Kyuss can move through/occupy the same space as another creature as if they weren't there, any creature that starts their turn in the same space as Kyuss must succeed on a DC 16 Dex save or have 5 (1d8) Kyuss worms latch onto their skin, on a success they are pushed out of the space Kyuss is occupying.

Diseased Life. Any spell that cures disease or removes a curse that is cast onto Kyuss will deal damage equal to 1d12 per spell level.

Creating Spawn. Any medium or small humanoid killed by Kyuss will rise again as a spawn of Kyuss at the start of their next turn. Additionally Kyuss may choose to swap out any number of undead for spawns of Kyuss when he casts spells that would animate/create undead. He may also turn any existing undead, that he chooses, within 100ft of him into spawns of Kyuss as an action(they keep their current hp unless that hp would be above the maximum hp of the spawn of Kyuss). Kyuss can command all of his spawn free of action and he can see and hear through them as long as they are in the same plane of existence of him.

Return To Worms. When Kyuss reaches 0hp he becomes 5(8, 10, or 15 depending on his size category when he dies) Swarms of Hunter Worms, these swarms will attempt to flee at all costs and if any one of them escape then Kyuss will reform in 2d10-(number of swarms which escaped) days, minimum of 1.

Demonic Soul. Demons or other creatures from the abyss recognise Kyuss as their leader instinctively and follow his commands (as such they do not make charisma saving throws when he summons them using spells such ass summon greater demon), if there is a reason that they would try to resist(such as if they were a party member who had poly morphed into a demon) then they can attempt to make a DC 20 wisdom save to resist the command, if successful they are immune to his commands for the next hour.

Legendary Resistances (3/day). When Kyuss fails a saving throw he can choose to succeed instead.