Asmodeus, Supreme Archduke of Nessus

Shapechanger. Asmodeus can change into any form he wishes. His statistics stay the same, save size.

Spellcasting. Asmodeus is a 20-level spellcaster. His spellcasting ability is Intelligence (spell save DC 23, +15 to hit with spell attacks). Asmodeus has following spells prepared:
Cantrips (at will): Acid splash, Fire bolt, Friends, Mage hand, Poison Spray

1st level (at will): Burning hands, Chromatic orb (necrotic, fire, poison, or cold damage only ), False life, Tasha's hideous laughter

2nd level (at will): Blindness/deafness, Crown of madness, Flaming sphere, Scorching ray

3rd level (at will): Animate Dead, Lightning Bolt, Fireball

4th level (at will): Banishment, Evard's black tentacles, Phantasmal Killer, Wall of fire

5th level (5 slots): Cone of cold, Modify memory, Wall of force

6th level (4 slots): Chain lightning, Circle of death, Flesh to stone

7th level (4 slots): Delayed blast fireball, Finger of death, Teleport

8th level (3 slots): Control Weather, Dominate monster

9th level (3 slots): Power word kill, Weird, Fire of the Nine Hells  (see my Homebrew spells)

Legendary resistance (6/Day). When Asmodeus fails a saving throw he can choose to succeed it instead.

Regeneration. Asmodeus gains 30 hit points at the start of his turn unless he takes 15 or more radiant damage. Asmodeus only dies if he starts his turn with 0 hit points and doesn't regenerate.

Multiattack. Asmodeus makes 9 attacks.

Flaming cleaver. Melee Weapon Attack: +20 to hit, reach 8 ft., 1 target. Hit: 46 (8d8 + 10) slashing damage 17 (1d12+10) fire damage. 

Fireball. Ranged Weapon Attack: +19 to hit, range 300/580 ft., 1 target. Hit: 81 (8d12 + 9) Fire damage.

Teleport. Asmodeus teleports to an unoccupied space within 120 ft.

Asmodeus can take 6 legendary actions, choosing from the options below. Only one legendary action can be used at a time and only at the end of another creature's turn. Asmodeus gains all spent actions on his turn.

Attack.  Asmodeus uses an action.

Spell. Asmodeus casts a spell requiring one action for every 3rd level of the spell (1st-3rd lv, one action, 4th-6th lv, two actions, 7th-9th lv 3 actions). Asmodeus does not need to use a spell slot if he has one for that level.

Regain spells. Asmodeus regains up to 6 spell slots, costing him one action for every spell slots regained. Every level above 5th costs an extra action.

Teleport. Asmodeus teleports to an unoccupied space within 120 ft.