Alastor Kall

Final Throw: When Alastor dies, he explodes. every creature within a 10 ft. radius sphere of him must make a DC 22 dexterity saving throw. a target takes 28 (8d6) fire damage on a failed save, or half as much damage on a successful save. 

power. Alastor's innate spellcasting ability is Strength (spell save DC 17). Alastor can cast the following spells, requiring no components:

At will: teleportation (30 feet), detect magic, fireball, hold monster, polymorph, wall of fire

1/year: wish

legendary save (2/day). if the (unknown) fails a saving throw, it can choose to succeed instead. 

glowing mind: detect thoughts of any kind.

Death. when the devil reaches 0 hitpoints, his second phase starts, turning him into an undead version of himself who can perform these actions/abilities. He starts at 150 hp when the second phase starts. Fireball, breath weapon, and flames.

flames (undead). every time Alastor damages a PC he gains another green glowing flame somewhere on his body which he can use in this action (fireball) 

Multiattack: The pit fiend makes 4 attacks: one with its bite, one with its claws, one with its tail, and one with its wings. it can replace the claw attack with one attack with its Ginat Slasher and can replace its tail attack with its constrict if it is grappling a creature.

Horizontal Gash. , Melee Weapon Attack: +10 to hit, reach 10 ft., multiple target hit: 14 (2d6 + 8)

Bite. Melee Weapon Attack: +15 to hit, reach 5 ft., one target Hit: 22 (4d6 + 8) piercing damage. The target must succeed on a DC 22 Constitution saving throw or become poisoned until cured by magic. While poisoned in this way, the target can't regain hit points, and it takes 21 (6d6) poison damage at the start of each of its turns.

Claw. Melee Weapon Attack: +15 to hit, reach 10 ft., one target Hit: 17 (2d8 + 8) slashing damage.

Tail. Melec Weapon Attack: +15 to Lit, reach 10 ft., one target. Hit: 24 (3d10 + 8) bludgeoning damage and the target is grappled (escape DC 23). Until this grapple ends, the creature is restrained and the pit fiend can't make a tail attack against another target.

Constrict. Melee Weapon Attack +15 to hit, reach 10 fL, a creature restrained by the pit fiend's tail. Hit: 24 (3d10 +8) bludgeoning damage.

Wing. Melee Weapon Attack +15 to hit, reach 10 ft., one target. Hit: 13 (2d4 + 8) bludgeoning damage.

earthquake. Alastor pummels his giant Slasher into the ground creating a shockwave dmg of (2d8) reaching a 12-foot radius from him.

breath weapon (undead). Alastor breathes green fire. +6 to hit, reach 10 feet, (2d20 + 2) acid dmg in a radius of 10 feet of the original target

fireball (undead) (Bonus) Alastor throws a green fireball off of his body, hit: +8, reach 60ft, damage: 2d10 + 3 damage

Over Here: when a melee weapon attack hits an ally within 5ft of Alastor, Alastor can make 1 Horizontal Gash attack against the creature that hit the ally 

Nourishment: When an ally dies within 60 ft of Alastor he can absorb their essence, regaining 3d8 hitpoint.

RISE!: ALasotr can summon 2d4 bearded devils and 1d4 barbed devils

pulse beams. sends a pulse beam of green fire from his eyes causing 4d20 of damage to one opponent 

Your useless: Alasotor can choose any ally of his choosing within 30ft of him to drain essence from and regain the same amount of hitpoint the ally had.