Tharizdun, the Chained Oblivion

Legendary Resistance (6/day). If Tharizdun fails a saving throw, he can choose to succeed instead.

Antimagic Cone. Tharizdun can create an area of antimagic, as in the antimagic field spell, in a 150-foot cone. At the start of Tharizdun's turn, he can choose which way the cone faces. Tharizdun can dismiss the cone or activate it as a bonus action. The cone doesn't function if Tharizdun is incapacitated.

Disincorporation. When Tharizdun drops to 0 hit points, his avatar form is destroyed, but his essence travels back to his prison just beyond the Multiverse. He is unable to take physical form for an unknown amount of time.

Limited Magic Immunity. Tharizdun is immune to spells of 5th level or lower unless he wishes to be affected. He also has advantage on saving throws against spells and spell-like effects.

Shapechange. Tharizdun can transform himself and any equipment he has into a form resembling any sentient creature of a small or larger size. Tharizdun can choose the make this form look however he wants, but all of his statistics remain the same in this new form. A creature can determine that the creature is Tharizdun by succeeding on a DC 27 Wisdom (Insight) or Wisdom (Religion) check.

Incorporeal Form (3/day). Tharizdun can, as a bonus action, turn himself incorporeal. While in this form, Tharizdun can move through solid objects and creatures as if they were difficult terrain and all attacks against Tharizdun have disadvantage. This effect lasts until the start of his next turn and if Tharizdun ends his turn inside of another object's space, he takes 55 (10d10) force damage that ignores damage resistance. and is immediately shunted into the nearest unoccupied space. While in this form he is immune to the grappled and restrained conditions for the duration.

Regeneration. Tharizdun regains 30 hit points at the start of his turn unless he is incapacitated or takes damage from a good-aligned creature.

Eldritch Weapons. Tharizdun's weapon attacks are magical, also when Tharizdun hits a creature with a weapon attack the creature that was hit takes an additional 38 (6d10) psychic damage (this damage is included in the attack).

Essence of Eldritch Magic. Tharizdun can concentrate on up to 3 spells at a time. If he casts another concentration spell while concentrating on 3 other spells, he can choose one spell he was concentrating on and end its effects immediately.

Innate Spellcasting. Tharizdun's spellcasting ability is Charisma (save DC 27, +19 to hit with spell attacks). Tharizdun can cast the following spells innately, requiring no spell components.

At will: shape water, mold earth, control flames, gust, toll the dead, eldritch blast (6 bolts, +10 to damage rolls), hold monster, lightning bolt, banishment, polymorph, fireball

3/day: divine word, disintegrate, plane shift, fire storm, chain lightning, prismatic spray, harm, geas, soul cage

1/day: feeblemind, reverse gravity, meteor swarm, power word kill, time stop

Multiattack.  Tharizdun can use his Frightful Presence. He then makes four attacks with his Weapon of Elemental Evil and one attack with his Shard of Voidharrow.

Summon Servants (1/day). Once per day, Tharizdun can summon demons and/or elementals of evil alignment with a combined total of 500 hit points to an unoccupied space, or several unoccupied spaces within 100 feet of him. These servants roll their own initiative and are friendly to Tharizdun and his allies. On future turns, Tharizdun can, as bonus action command the servants to do certain actions, which the servants must do exactly in the way Tharizdun intended.

Song of Chaos (Recharge 6). Tharizdun utters a song that twists and warps the very sanity of the universe. Each creature within 240 feet of Tharizdun must make a DC 27 Charisma saving throw or take 77 (22d6) psychic damage and gain a form of indefinite madness that lasts until the creature is restored through spells like greater restoration.

Frightful Presence. Each creature of Tharizdun’s choice that is within 240 feet of Tharizdun and aware of him must succeed on a DC 27 Wisdom saving throw or become frightened for 1 minute. 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 Tharizdun's Frightful Presence for the next 24 hours.

Weapon of Elemental Evil. Melee Weapon Attack: +19 to hit, reach 15 ft., 1 target. Hit: 37 (4d12 + 10) acid, fire, cold, or bludgeoning (Tharizdun's choice) damage plus 38 (6d10) psychic damage. 

Shard of Voidharrow. Melee Weapon Attack: +15 to hit, reach 15 ft., 1 target. Hit: 23 (4d6+6) piercing damage plus 38 (6d10) psychic damage. A creature slain by this weapon rises as an abyssal wretch under Tharizdun's control after one round.

Maddening Blast. Ranged Spell Attack: +19 to hit, range 300 ft., 1 target. Hit: 92 (17d10) psychic damage and the target must make a DC 27 Charisma saving throw or be inflicted with a form of short term madness.

Wild Magic Surge. If Tharizdun sees a spell being cast, he can, as a reaction, cause a wild magic surge centered on the caster. When the wild magic surge occurs, the spell being cast is replaced by a random d100 effect on the wild magic table found in the sorcerer section of the PHB.

Tharizdun can take 4 legendary actions, choosing from the list below. Only one legendary action option can be used at a time and only at the end of another creature's turn. Tharizdun regains spent legendary actions at the start of his turn.

At-Will Spells. Tharizdun casts a spell that he can cast at-will

Drain Life (Costs 2 Actions). Tharizdun unleashes a wave of life-draining destruction. Each creature within 60 feet of Tharizdun must make a DC 27 Constitution saving throw or take 44 (8d10) necrotic damage on a failed save or half as much on a success. Tharizdun regains hit points equal to half the damage dealt.

Legendary Action Mimicry (Costs 3 Actions). Tharizdun can mimic the legendary actions that are available to any demon lord, prince of elemental evil, or other creature associated with Tharizdun (DM's discretion).

Shard Attack. Tharizdun makes an attack with his Shard or Voidharrow.