Regeneration. Tharizdun, the Chained Oblivion regains 50 hit points at the start of each of his turns.

Creator of Chaos. Withing 1000 feet of Tharizdun, the Chained Oblivion, all wild magic surge chances increase by one for every level of that spell.

Duke of Madness. Any creature that can see Tharizdun, the Chained Oblivion must make a DC 22 Wisdom saving throw. On a failure, the creature takes 16 (4d6) psychic damage and gains one form of indefinite madness (DMG pg. 260). On a success, the creature takes half damage and does not become maddened. A creature immune to fear is immune to this ability.

Spellcasting. Tharizdun, the Chained Oblivion is a 20th-level spellcaster. Its spellcasting ability is Intelligence (spell save DC 22, +14 to hit with spell attacks). Tharizdun, the Chained Oblivion has the following wizard spells prepared:
Cantrips (at will): Fire Bolt, Ray of Frost, Shape Water, Toll the Dead, Frostbite
1st level (6 slots): Chromatic Orb, Ray of Sickness
2nd level (5 slots): Mirror Image, Hold Person, Crown of Madness
3rd level (5 slots): Vampiric Touch, Summon Lesser Demon, Haste
4th level (5 slots): Summon Greater Demon, Storm Sphere, Phantasmal Killer, Gravity Sinkhole, Dimension Door, Blight
5th level (5 slots): Telekinesis, Planar Binding, Negative Energy Flood, Infernal Calling (Replace devil with Demon), Far Step, Dominate Person, Cone of Cold
6th level (4 slots): Scatter, Mental Prison, Gravity Fissure, Flesh to Stone, Eyebite, Disintegrate, Arcane Gate, Circle of Death, Chain Lightning
7th level (4 slots): Reverse Gravity, Power Word Pain, Prismatic Spray, Forcecage, Finger of Death, Crown of Stars
8th level (3 slots): Reality Break, Maze, Illusory Dragon, Feeblemind
9th level (3 slots): Time Ravage, Power Word Kill, Foresight, Gate, Imprisonment, Meteor Swarm

Multiattack. Tharizdun, the Chained Oblivion may use it's contorted madness. It then makes three attacks: one with its slam, one with its deathly stare, and one with its chain whip.

Slam. Melee Weapon Attack: +19 to hit, reach 10 ft., one target. Hit: 16 (1d10 + 10) necrotic damage and 17 (2d10 + 5) psychic damage.

Chain Whip. Melee Weapon Attack: +19 to hit, reach 20 ft., one target. Hit: 17 (1d12 + 10) bludgeoning damage.

Deathly Stare. Tharizdun, the Chained Oblivion unleashes a stare that instills doom and fear into one creature within 120 ft that Tharizdun, the Chained Oblivion can see. That creature must make a DC 22 Wisdom saving throw or take 23 (3d10 + 5) psychic damage and become frightened of Tharizdun, the Chained Oblivion for one minute. The frightened creature may make this saving throw again at the end of each of their turns. A creature that succeeds this saving throw is immune to Tharizdun, the Chained Oblivion's deathly stare for 24 hours.

Tharizdun, the Chained Oblivion can take 5 legendary actions, choosing from the options below. Only 1 legendary action option can be used at a time and only at the end of another creature's turn. Tharizdun, the Chained Oblivion regains spent legendary actions at the start of its turn.

Deathly Stare. Tharizdun, the Chained Oblivion uses its deathly stare.

Attack. Tharizdun, the Chained Oblivion makes either one slam or one chain whip attack.

Move. Tharizdun, the Chained Oblivion moves up to his movement.

Spell Casting (Costs 2 Actions). Tharizdun, the Chained Oblivion casts one spell.