Azakath Herald of Asmodeus

Fear Aura. Any creature hostile to Azakath that starts its turn within 20 feet of him must make a DC 21 Wisdom saving throw, unless he 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 the pit fiend's Fear Aura for the next 24 hours.

Devil of the Nine Hells. Any creature that moves within 5 feet of Azakath for the first time on a turn or ends its turn there takes 1d10 fire damage. 

Legendary Resistance 3/day. If Azakath fails a saving throw, he can instead choose to succeed.

Magic Weapons. Azakath's weapon attacks are magical.

Innate Spellcasting. Azakath's spellcasting ability is Charisma (spell save DC 21). He can innately cast the following spells, requiring no material components:

At will: detect magic, fireball

3/day: dominate person, wall of fire

2/day: flame strike, delayed blast fireball, fire storm

1/day: banishment, meteor swarm

Multiattack. Azakath makes three attacks: One with his scepter and two with his claw.

Scepter. Melee Weapon Attack: +10 to hit, reach 10 ft., one target. Hit: 15 (2d6 + 8) bludgeoning damage plus 21 (6d6) fire damage.

Claw. Melee Weapon Attack: +10 to hit, reach 10 ft., one target. Hit: 17 (2d8 + 8) slashing damage.

Summon Devil (2/Day). Azakath chooses what to summon and attempts a magical summoning.

Azakath summons 2d4 bearded devils, 1d4 barbed devils, or one erinyes with no chance of failure.

A summoned devil appears in an unoccupied space within 60 feet of its summoner, acts as an ally of its summoner, and can’t summon other devils. It remains for 1 minute, until it or its summoner dies, or until its summoner dismisses it as an action.

Azakath can take 2 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. He regains spent legendary actions at the start of its turn.

Investiture of Flame (1 Action). Azakath creates a line of fire 15 feet long and 5 feet wide extending from him in a direction you choose. Each creature in the line must make a Dexterity saving throw. A creature takes 4d8 fire damage on a failed save, or half as much damage on a successful one. The line lasts until the end of his next turn. Any creature that moves through the fire must make a Dexterity saving through or take 2d8 fire damage on a failed save or half as much on a successful one.

Bane of the Great Hero (2 Actions). Azakath's voice echoes through the battlefield; all enemy creatures must make a DC 21 Wisdom saving throw or be frightened until the end of their next turn. All allied creatures are empowered and have advantage on attack rolls against frightened creatures.