Innate Spellcasting. Avatar of Kyuss’s spellcasting ability is Intelligence (spell save DC 22). It can innately cast the following spells requiring no material components

3/Day: cone of cold, counterspell, dispel magic, greater invisibility, haste

1/Day: mislead, time stop, weird

One With Time and Space. Avatar of Kyuss has advantage on initiative rolls and adds +5 to the result.

A Bleed In Time and Space. After a long period of combat with Avatar of Kyuss, the time and space around it begins to warp and tear as past, present and future versions of the creatures involved in the battle begin to wink in and out of existence. At the beginning of each player’s turn after Round 10, they must roll a d100 and on a result of 74 or lower, choose a second player at random. Both players must then roll a DC 22 Dexterity saving throw or take half their current total hit points in force damage as the matter of their bodies occupy the same space as another, suddenly appearing in that space. On a successful save, players take no damage as they successfully avoid colliding with another.

At the beginning of each player’s turn after Round 15, choose a second player at random. Both players take half their current total hit points in force damage as the sheer amount of past, present and future versions of themselves becomes too great to avoid collision.

Immutable Form. Avatar of Kyuss is immune to any spell or effect that would alter its form  

Legendary Resistance (3/Day). Avatar of Kyuss fails a saving throw, it can choose to succeed instead.

Gravitational Force. Avatar of Kyuss's melee attacks count as magical.

Magic Resistance. Avatar of Kyuss has advantage on saving throws against spells and other magical effects. 

Multiattack. Avatar of Kyuss makes four attacks: two with its mace, one with its worm tentacle and one with its dimension ray.

Mace. Melee Weapon Attack. +16 to hit, reach 20 ft, one target. Hit: 23 (3d10+7) bludgeoning damage.

Worm Tentacle. Melee Weapon Attack. +16 to hit, reach 120 ft, one target. Hit: 35 (8d6+7) psychic damage and the target must succeed on a DC 22 Constitution saving throw or its hit point maximum is reduced by an amount equal to the damage taken. This reduction lasts until the target finishes a long rest.

Dimension Ray. Ranged Weapon Attack. +16, reach 300 ft, one target. Hit: 58 (13d8) psychic damage and the target must succeed on a DC 30 Charisma saving throw. On a failed save, the target must immediately take a full turn of action, bonus action, movement and/or free object interaction which occurs 1d4 rounds later, replacing their turn in the initiative order as they are caught in a tear in time and space.

Spell slots, consumable items, limited use class features and other expendable actions used in this way are depleted as normal but do not take effect or trigger until the replaced round later in round order. The trigger for the target’s nominated course of action still occurs regardless of how the layout of the battlefield has changed or the locations of their allies in relation to them and their choices.

If the target has not moved in the time leading up to their replaced turn in the following initiative rounds, the target takes half its current hit point total in force damage as a version of themselves appears from a tear in time and space to occupy the same location they are currently in. 

On a successful save, the target takes half the total damage and none of the effects. 

Avatar of Kyuss 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. Avatar of Kyuss regains spent legendary actions at the start of its turn.

Fifth Corner In A Square Room (Costs 1 Action). Avatar of Kyuss uses its Dimension Ray without any of the damage.