Azemous

Keeper of souls. When azemous kills a creature he takes their soul and adds it to his collection.

Power from souls. Azemous starts with 22 souls and gains benifits and abilities using souls. At the start of each of his turns he gains temp hp equal to the number of souls in his possession.

Soul casting. Azemous may cast his available spell at any spell leveling, abiding by the original level of the spell, using souls. Spells casted this way uses a number of souls equal to the level of the spell. 

Soul power. Azemous can use a soul to roll saving checks with true advantage. 

Soul power. When azemous deals damage to a creature he may use a soul and have them roll a DC 23 con save, on a fail they take additional damage equal to the amount of souls in his possession.

Regeneration. Azemous regains 20 hit point at the start of each of his turns, taking radiant damage between turns prevents this Regeneration. Azemous only dies when his hitpoints are at 0 and he cannot Regenerate.

Spellcasting. Azemous is a 7th-level spellcaster. Its spellcasting ability is Charisma (spell save DC 23, +15 to hit with spell attacks). Azemous has following Warlock spells prepared:
6th level (8 slots): cause fear, hellish rebuke, hex, darkness, hold person, misty step, counterspell, spirit shroud, vampiric touch, blight, enervation, infernal calling, synaptic static, circle of death, soul cage, inflict wounds, vortex warp, pulse wave, vitriolic sphere, destructive wave.

Multi-attack. Azemous on his turn can make 3 attacks, 3 claw or tail stabs or 2 claw or tail stabs and a tail slam, or a bite

Claw. Melee Weapon Attack: +16 to hit, reach 10 ft., Hit (3d12 + 9) [magical slashing] damage + 4d6 necrotic damage. 

Bite. Melee Weapon Attack: +16 to hit, reach 5 ft., Hit (7d6 + 9) [necrotic] damage. He can either heal health equal to the damage dealt or force the creature to make a dc22 con save, on a fail they gain 1 level of exhaustion.

Tail Stab. Melee Weapon Attack: +16 to hit, reach 15 ft., Hit (3d12 + 9) [magical piercing] damage + 4d6 necrotic damage. 

Tail slam. Melee Weapon Attack: +16 to hit, reach 15 ft., Hit (5d12 + 9) [magical bludgeoning] damage. When a creature is hit by this attack, they must make a DC 23 strength saving throw or be thrown 25ft away. Or Azemous may instead choose to grapple with his tail, if the creature is large or smaller, they are restrained and lifted 15ft off the ground when they break out, they fall 15ft and become prone. When a creature is grappled by his tail, he can no longer tail stab though and the tail slam range is reduced by 5ft and the creature in the grapple takes half the damage dealt by the slam.

Azemous can use his powerful wings to give himself a burst of speed. He can move 15ft In any direction.

When Azemous casts a spell with the casting time of an action he may use his bonus action to make an attack.

Fiendish mind. When Azemous is subjected to a spell or effect that makes him roll a int, wis or cha save he forces the creature to make a dc 23 wisdom save on a fail they take 8d8 psychic damage or half as much on a success. If he succeeded the save he can use a soul to force the creature to automatically fail the save.

Azemous has 3 legendary actions

Attack. Azemous can make a single attack from his attack list or cast a spell from his spell list.

Mass Harvest (costs 3 actions). Every living creature with equal to or less than 50 hit points within 50 ft of Azemous must make a DC 23 con save, on a failed save azemous steals their soul and they die instantly a wish spell is the only way to revert this death.

Portal into hell (costs 2 actions). Azemous can create a Medium Portal to hell within 30ft of him with this Portal he can do one of 3 things. Summon 22 Imps which attack any creature that azemous can see at his command, if the imps kill any creature they bring the soul to azemous. He can force all creatures within 15ft of the Portal to stare into it, the creatures make a DC 23 wisdom save, on a fail they take 6d12 psychic damage and is incapacitated until the start of their next turn, on a success they take half as much damage and is not incapacitated. He can cast the Plane Shift spell.

Overpower (costs 3 actions, recharge 1 day). Azemous enters a straining for full of power. He becomes gargantuan in size increases the range of all melee attacks by 5ft. heals 1/4 of his health any over healing transfers to temp hp. when azemous rolls for damage he takes the average roll or higher. at the end of each of its turns it makes a dc 26 con save.