Ascended Avarus Tyrannus

Diabolical Restoration. If the devil dies outside the Nine Hells, its body disappears in sulfurous smoke, and it gains a new body instantly, reviving with all its Hit Points somewhere in the Nine Hells.

Magic Resistance. The devil has Advantage on saving throws against spells and other magical effects.

Legendary Resistance. Avarus has 6 Legendary resistances that reset at the start of each day.

Legendary Actions. Avarus has 6 Legendary actions that recharge at the start of each day.

Tethered to Reality. Avarus Tyrannus is immune to the effects of Banishment, and [Tooltip Not Found], unless he consents.

Multiattack. The devil makes three attacks, using Flame Tongue, Hellish Lightningl, or Hurl Flame in any combination. 

Flame Tongue. Melee Attack Roll: +8 to hit, and on a hit deals 3d6+5 Slashing damage and 2d8+8 Fire damage.

Hurl Flame. Ranged Attack Roll: +8, range 150 ft. Hit: 26 (5d8 + 8) Fire damage. If the target is a flammable object that isn’t being worn or carried, it starts [rules]burning[/rules. 

Hellish Lightning. Ranged Attack Roll: Avarus targets two creatures he can see, and call down bolts of red, infernal lightning on them. Each creature targeted must make a DC 17 Dexterity save, or take 5d10 Lightning Damage.

Spellcasting: Avarus Tyrannus can cast the following spells, using Charisma as the spellcasting ability and without material components. (Spell save DC 18)

At Will: Fire Bolt, Invisibility, Command, Mage Hand, Detect Thoughts, Vicious Mockery, Fireball

5/Day: Greater Invisibility, Misty Step, Hold Person (level 5), Fireball (6th level), Lightning Bolt (6th Level), Silence, Wall Of Fire

3/Day: Dominate Monster, Dominate Person

2/Day: Antilife Shell, Power Word Kill, Meteor Swarm

Ray of Power. Avarus fires a heavily concentrated beam of raw force at a target he can see within 120 feet. The target takes 10d10 Force damage, and half as much on a successful DC 18 Wisdom saving throw. For every 100 hit points the target has, they take an additional 40 Force damage.

Infernal Teleport. Avarus Tyrannus teleports to an unoccupied space he can see within 120 feet. When he enters an area, every creature in a 15 foot cube centered on himself takes 3d8 Fire damage.

Summon. Summons 1d6 Cambions. This recharges after 1d4 rounds.

Counterspell. Avarus Tyrannus casts Counterspell in response to that spell’s trigger, using the same spellcasting ability as Spellcasting.

3/Day. Shield. Avarus Tyrannus can cast Shield in response to the spell's trigger, using the same spellcasting ability as Spellcasting. 

Damage Manipulation. Avarus Tyrannus can change the damage type of an incoming attack to Fire, and heals half the amount of damage he would have taken.

mage Manipulation. hg 

Teleport. Avarus Tyrannus teleports to an unoccupied space he can see within 120 feet.

Summon. Summons 1d4 Cambions, or 2d4 Cultist Fanatics. This recharges after 1d4 rounds.

Slash. Make a Flame Tongue attack, then cast an At Will spell.