Arkkos

Shapechanger. Can use an action or reaction to transform into a dark, shapeless mass or back into its solid form. The shapeless form can only be used to avoid attacks or traverse great distances.

While in shapeless form, it can’t speak and it has a flying speed of 60 feet. Its statistics, other than its size and speed, are unchanged. Anything it is wearing transforms with it, but nothing it is carrying does. It reverts to its solid form if it dies.

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

Regeneration. It regains 15 hit points at the start of its turn if it has at least 1 hit point. If it takes radiant damage or damage from holy water, this trait doesn’t function at the start of the its next turn.

Multiattack. (Solid Form Only). Makes three attacks, only one of which can be a Sullen Strike attack.

Void Missile (Solid Form Only). Ranged Magical Attack: +9 to hit, reach 120 ft. Hit: 7 (1d6+5) necrotic damage. Creates five black darts of magical force. Each dart hits a creature of choice that is seen within range. The darts all strike simultaneously, and can be directed to hit one creature or several.

Sullen Strike (Solid Form Only). Finesse Melee Attack: +7 to hit, reach 15 ft. Hit: 13 (1d12+8) slashing damage or Dexterity saving throw of 14. Upon a failed save, become grappled by inky, black tendrils. On a success, take damage instead. 

Misery Clasp (Only if Grappling). Psionic Magical Attack: Whilst grappling a creature, it stares into that creature's eyes, attempting to hypnotize it. That creature must then make a Wisdom saving throw of 19 or come under the charm spell, doing whatever it tells them to. Can break free of the mind control by reattempting the Wisdom save on the end of their turn. 

Can take 1 legendary action at the end of another creature’s turn. Regains spent legendary actions at the start of its turn.

Move. Moves up to its speed without provoking opportunity attacks.

Can take 1 mythic action once per day. Regains spent mythic action points at the start of the next day. 

Doom Desire: Creates a momentary 10-foot-radius sphere of dark, void-like space that corrupts all those caught within its range. Each creature caught in the sphere must make a Constitution saving throw. A target takes 10d12 necrotic damage on a failed save, or half as much damage on a successful one. The creature with the most maximum hit points will also make a Wisdom saving throw of 17, and on a failed save will become a minion of Arkkon, permanently transformed into a shadow of its former self. On a successful save, take the half damage. The sphere only appears for an instant, and retracts within seconds.