Axoruul, The Great Disarray

Relentless Disarray (Mythic Trait; Recharges after a Short or Long Rest). When Axoruul is reduced to 0 hit points, he doesn't die or fall unconscious. Instead, the damage destabilizes his form, he regains 450 hit points and his walking speed becomes 60.

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

Chaos Aura. Whenever a creature within 50 feet of Axoruul casts a spell of 8th level or lower, it must make an Intelligence (arcana) check. On a 5 or lower, the spell fails, is wasted and the caster takes 4d8 necrotic damage. On a 10 or lower, the spell fails, is wasted and the caster must succeed on a DC 18 Wisdom saving throw or 3d8 necrotic damage. On a 15 or lower, the spell fails and is wasted. On anything higher than a 15, the spell functions as intended.

Incomprehensible Nature. Axoruul doesn't require air, food, drink or sleep, no magic can read his thoughts or emotions and any creature that communicates with Axoruul telepathically must succeed on a DC 18 Wisdom saving throw or take 6d8 psychic damage.

Immutable Form. Axoruul is immune to any magic that would alter his form.

Magic Resistance. Axoruul has advantage on saving throws against magic.

Magic Weapons. Axoruul's weapon attacks are magical.

Multiattack. Axoruul makes three melee or ranged attacks.

Chaos Slam. Melee Weapon Attack: +19 to hit, reach 15 ft., 1 target. Hit: 25 (3d10 + 10) bludgeoning damage + 14 (4d6) necrotic damage. The target must succeed on a DC 18 Strength saving throw or be pushed 10 feet in a random direction and knocked prone.

Disrupting Lance. Ranged Weapon Attack: +19 to hit, range 100/500 ft., 1 target. Hit: 23 (3d8 + 10) force damage + 14 (4d6) necrotic damage. If the target is a creature, it must succeed on a DC 18 Constitution saving throw or be stunned until the end of its next turn.

Tides of Chaos (Recharge 6). Axoruul emits a blast of chaotic energy in a 90-foot cone. Each creature in that area must make a DC 21 Wisdom saving throw, taking 45 (7d12) psychic damage and 45 (7d12) necrotic damage on a failed save, half that amount on a successful one. Any objects (magical or otherwise) that aren't being worn or carried are corrupted by chaos and loose any properties they previously held. Medium or smaller objects are destroyed by this effect.

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

Move. Axoruul moves up to half his speed in any direction.

Attack. Axoruul makes one melee or ranged attack.

Destabilize (Costs 2 Actions). Axoruul warps a large or smaller object that he can see within 30 feet of him for 1 round. As a bonus action, Axoruul can move the object 20 feet in any direction. Any creature that ends its turn within 10 feet of the object takes 22 (5d8) necrotic damage and is stunned until the end of its next turn. The object is destroyed when this effect ends.

If Axoruul's mythic trait is active, he can use the options below as legendary actions for 1 hour after using Relentless Disarray.

 Wrathful Blows. Axoruul makes two melee or ranged attacks.

Chaos Storm (Costs 2 Actions). Axoruul summons a storm of chaotic energy in a 15 by 15 foot area that he can see within 50 feet him. The storm lasts for three rounds and only one storm can be active at a time. The storm counts as difficult terrain and its area is heavily obscured. Any creature that ends its turn within 10 feet of the storm is pulled into the storms area and takes 18 (4d8) necrotic damage at the start of each of its turns while in the storm. The creature can escape the storm by making a successful DC 18 Dexterity saving throw.

Chaos Burst (Costs 3 Actions). Axoruul targets one creature that he can see within 30 feet of him. That creature must succeed on a DC 18 Constitution saving throw or take 54 (12d8) necrotic damage. If this damage reduces a creature to 0 hit points, Axoruul kills it by blasting it into shining particles of energy.