Ahz'Araath the Star-Eater

Ravenous. Once per day, when a creature is reduced to 0 hit points by Ahz'Araath, a 5-inch diameter singularity appears within them that devours them from the inside. A creature killed by this singularity leaves no remains and cannot be brought back to life by any means short of a wish spell. Ahz'Araath then regains hit points equal to half the devoured creature's hit point maximum. If another creature touches the singularity, they are instantly killed the same way. This singularity is stationary and lasts for a minute before disappearing. 

Legendary Resistance (4/Day). If Ahz'Araath fails a saving throw, he can choose to succeed instead.

Alien Mind. If a creature tries to read Ahz'Araath's thoughts or deals psychic damage to him, that creature must succeed on a DC 23 Intelligence saving throw or be stunned for 1 minute. The stunned creature can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success.

Eldritch Horror. Ahz'Araath's abilities that would frighten a creature ignore immunity to the frightened condition.

Multiattack. Ahz'Araath can use its Eldritch Fright. It then makes three attacks: one with its bite and two with its claws, each of which it can replace with one use of it's Tentacle or Fling.

Bite. Melee Weapon Attack: +15 to hit, reach 10 ft., one target. Hit: 19 (2d10 + 8) piercing damage plus 3 (1d6) psychic damage.

Claw. Melee Weapon Attack:+15 to hit, reach 10 ft., one target. Hit: 14 (2d8+5) slashing damage plus 3 (1d6) psychic damage.

Tentacle. Melee Weapon Attack: +15 to hit, reach 30 ft., one target. Hit: 20 (3d6 + 10) bludgeoning damage, and the target is grappled (escape DC 18). Until this grapple ends, the target is restrained. Ahz'Araath has ten tentacles, each of which can grapple one target.

Fling. One Large or smaller object held or creature grappled by Ahz'Araath is thrown up to 60 feet in a random direction and knocked prone. If a thrown target strikes a solid surface, the target takes 3 (1d6) bludgeoning damage for every 10 feet it was thrown. If the target is thrown at another creature, that creature must succeed on a DC 18 Dexterity saving throw or take the same damage and be knocked prone.

Eldritch Fright. Each creature that is within 120 feet of Ahz'Araath and aware of him must succeed on a DC 23 Wisdom saving throw or become frightened for 10 minutes. 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 Ahz'Araath's Eldritch Fright for the next minute.

Black Blizzard (Recharge 5-6). Ahz'Araath unhinges his jaw and a massive worm-like creature writhes itself out of Ahz'Araath, curls up while floating above him, and collapses into a spinning singularity that lasts until the end of Ahz'Araath's next turn. All creatures that can see this undergo the effects of Eldritch Fright. Additionally, a 100 ft. black storm swirls around the singularity. Ranged weapon attacks and flight are impossible in the storm and the area is heavily obscured. At the end of every turn taken while the storm is active, all creatures in the storm must make a DC 18 strength saving throw or be pulled 5 feet closer to the singularity. If a creature is pulled within 5 feet of Ahz'Araath and the singularity, they take 33(6d10) force damage and are teleported back 30 feet. A creature cannot move away from the singularity using movement- and moving adjacent to it counts as difficult terrain. All saving throws made by creatures within the storm are made with disadvantage. Ahz'Araath is completely immune to the effects of this storm but the singularity is always centered above him.

Slam. As a reaction to a creature failing to escape his tentacle's grapple, Ahz'Araath can slam the grappled creature into a hard surface before ending the grapple, dropping the creature prone. The creature takes 22 (5d8) bludgeoning damage and must succeed a DC 16 Constitution saving throw or be knocked unconscious until another creature uses an action to shake them awake. The unconscious creature can repeat the saving throw if it takes damage, ending the effect on itself on a success.

Ahz'Araath 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. Ahz'Araath regains spent legendary actions at the start of his turn.

Tentacle Attack or Fling. Ahz'Araath makes one tentacle attack or uses his Fling.

Wing Attack (Costs 2 Actions). Ahz'Araath sprouts two large wings and beats them. Each creature within 15 feet of Ahz'Araath must succeed on a DC 23 Dexterity saving throw or take 15(2d6 + 8) bludgeoning damage and be knocked prone. 

Frighten Foes (Costs 2 Actions). Ahz'Araath targets up to three creatures he can see within 30 feet of him. Each target must succeed on a DC 23 Wisdom saving throw or be frightened of him until the end of his next turn. Any target within 5 feet of Ahz'Araath has disadvantage on the saving throw.

Azathoth's End (Costs 3 Actions). All magical effects in a 120-foot-radius centered on Ahz'Araath end as if by the antimagic field spell and all frightened creatures in this radius take 36 (8d8) psychic damage.