Asmodeus, Lord of the Ninth

Demands of Nessus. At the start of each of his turns, Asmodeus can choose one ally he can see within 30 feet of himself. The chosen ally loses 10 hit points, and Asmodeus regains the same number of hit points. If Asmodeus is incapacitated, he makes no choice; instead, the closest ally within 30 feet is the chosen ally.

Devil's Sight. Magical darkness doesn't impede Asmodeus's darkvision.

Fear Aura. Any creature hostile to Asmodeus that starts its turn within 20 feet of him must make a DC 27 Wisdom saving throw, unless Asmodeus is incapacitated. On a failed save, the creature is frightened until the start of its next turn. If a creature's saving throw is successful, the creature is immune to Asmodeus’ Fear Aura for the next 24 hours.

Hellish Weapons. Asmodeus's weapon attacks are magical. Whenever he hits with any weapon, the attack does an extra 55 (10d10) necrotic damage (included in the attacks below).

Innate Spellcasting. Asmodeus's spellcasting ability is Charisma (spell save DC 27, +16 to hit with spell attacks). He can innately cast the following spells, requiring no material components: 

At will: alter self (can become Medium when changing his appearance), blight, dominate person, dominate monster, fireball, hellish rebuke (8th level), planar binding, plane shift (other creatures that Asmodeus wishes are considered willing for the purpose of the spell), ray of frost (4d8 cold damage), teleport, wall of fire

3/day each: earthquake, weird

1/day each: imprisonment, meteor swarm, wish

Limited Magic Immunity. Asmodeus is not effected by spells of 6th level or lower unless he wishes to be. He has advantage on saving throws against all other spells and other magical effects.

Legendary Resistance (5/Day). If Asmodeus fails on a saving throw, he can choose to succeed instead.

Regeneration. Asmodeus regains 40 hit points at the start of his turn. If he takes radiant damage, this trait doesn't function at the start of his next turn. Asmodeus dies only if he starts his turn with 0 hit points and doesn't regenerate.

Spellcasting. Asmodeus is a 20th-level spellcaster. His spellcasting ability is Charisma (spell save DC 27, +19 to hit with spell attacks). Asmodeus knows the following sorcerer spells:
Cantrips (at will): blade ward, fire bolt, message, minor illusion, prestidigitation, shocking grasp

1st level (4 slots): shield, silent image

2nd level (3 slots): enhance ability, knock

3rd level (3 slots): blink, slow

4th level (3 slots): banishment, ice storm

5th level (3 slots): creation, teleportation circle

6th level (2 slots): disintegrate, globe of invulnerability

7th level (1 slot): prismatic spray

8th level (1 slot): earthquake

9th level (1 slot): time stop

Multiattack. Asmodeus makes four melee attacks.

Ruby Rod (Maul). Melee Weapon Attack: +22 to hit, reach 10 ft., one target. Hit: 25 (4d6 + 13) bludgeoning damage plus 55 (10d10) necrotic damage.

Death Blast. Ranged Spell Attack: +19 to hit, range 30/160 ft., one target. Hit: 55 (10d10) necrotic damage.

Teleport. Asmodeus magically teleports, along with any equipment he is wearing and carrying, up to 120 feet to an unoccupied space he can see.

Asmodeus can take 5 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 regains all spent legendary actions at the start of his turn.

None May Oppose Me! (Costs 2 Actions). Asmodeus forces up to seven creatures he can see to make a DC 27 Wisdom saving throw. On a failed save, the creature must spend each of its next four turns taking the Dash action to get as far away from Asmodeus as possible.

Ruby Rod. Asmodeus makes one Ruby Rod attack.

Teleport. Asmodeus uses his Teleport action.