Discorporation. When Mizneria drops to 0 hit points or dies, her body is destroyed but her essence travels back to her domain in the Nine Hells, and she is unable to take physical form for a time. However, if she is killed in her domain, this does not happen.

Innate Spellcasting Mizneria's innate spellcasting ability is [Intelligence] (Spell save DC 30, +25 to hit). Mizneria can cast the following spells without needing components or expending a spell slot (I would suggest choosing a smaller list to run specifically for your game):

At will: [Animate dead], [geas] (9th level), [ice storm] (9th level), [thunderwave] (9th level), [fire bolt] (max level), [chil touch] (max level), [bestow curse] (9th level), [fireball] (9th level), [chain Lightning] (9th level), [disguise self], [identify], [blight] (6th level), [phantasmal force].

4/day each: [Blindness/deafness] (9th level), [enthrall], [dispel magic] (9th level), [counter spell] (9th level), [slow] (unlimited range), [hold monster], [mass suggestion] (9th level), [wall of fire], [wall of ice], [sickening radiance], [polymorph].

3/day each: [Banishment], [phantasmal killer], [dominate person] (9th level), [mental prison], [harm], [finger of death], [forcecage], [divine word], [greater invisibility], [symbol].

2/day each: [Feeblemind], [dominate monster] (9th level), [shapechange], [imprisonment], [power word kill], [true polymorph], [mass polymorph].

1/day each. [Dark star], [true resurrection], [meteor swarm]

Limited Magic Immunity. Mizneria is immune to spells of the 6th level or lower, unless he wants to be affected.

Regeneration. Mizneria regains 30 hit points at the start of her turn as long as she hasn't taken any radiant damage.

Magic Weapons. Mizneria's weapon attacks are magical.

Legendary Resistance (5/day). If Mizneria fails a saving throw, she can choose to succeed instead.

Magic Resistance. Mizneria has advantage on saving throws against magical effects.

Reverie of Hell. As an action, Mizneria can encase herself in a protective barrier which heals her and stops enemies from attacking her. This barrier acts as a globe of invulnerability. The Reverie of Hell lasts for three rounds, with a different effect for each one:

• In the 1st round, Mizneria is automatically purged of any unwanted spell effects of conditions.
• In the 2nd round, Mizneria is purged of any diseases, poisons, or physical maladies (including lost body parts).
• In the 3rd round, Mizneria is healed to full hit points and feels as though she just rested a full day, regaining spells and class abilities accordingly.

Multiattack. Mizneria can use her frightful presence. She then makes five attacks with either her Burning Sickle or a slam, and then one with her horns.

Slam. Melee Attack: +14 to hit, reach 10 ft., one target. Hit: 53 (10d8+8) bludgeoning damage.

Burning Chain Scythe. Melee Weapon Attack: +17 to hit, reach 10 ft., one target. Hit: 51 (8d8 + 15 ) piercing damage. Hit: 51 (10d8+6) fire damage.

Throw Burning Chain Scythe. Ranged Weapon Attack: +14 to hit, range 150 ft., one target. Hit: 50 (9d8 + 10) piercing damage and Hit: 51 (10d8+6) fire damage. Has the Returning Property. Mizneria can use one of her attacks on the return of the sickle

Horns Melee Attack: +14 to hit, Hit: 10 (4d4) piercing damage. The target must succeed on a DC 20 Charisma saving throw or get poisoned. The target must roll again every day. If they fail the saving throw, they take 11 (3d10) poison damage. If they succeed, they are no longer poisoned.

Frightful Presence. Each creature of Mizneria's choice that is within 200 feet of Mizneria and aware of him must succeed on a DC 30 Wisdom saving throw or become frightened for 1 minute. A creature can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success. If a creature’s saving throw is successful or the effect ends for it, the creature is immune to Mizneria's Frightful Presence.

Innate Spellcasting. As A bonus Action, Mizneria can innately cast one spell.

Soul Bartering. Mizneria can grant any humanoid one wish (using the spell wish) in its lifetime on the condition that the PC is bound to him after death. Mizneria can make minor changes to the wishes if he wants. Mizneria suffers no harmful effects from this action.

Innate Spellcasting. As A bonus Action, Mizneria can innately cast one spell.

"Now you see me". As a bonus action on his turn, Mizneria can teleport anywhere within his line of sight or in the nine hells.

Loyalty to the Lady. As a bonus action, if Mizneria's Burning Scythe was ever knocked or taken out of her hand, she can instantly teleport it back into her palm

The Mizneria can take 5 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. The Mizneria regains spent legendary actions at the start of its turn.

Burning Chain Scythe (Costs 1 Action). Melee Weapon Attack: +17 to hit, reach 10 ft., one target. Hit: 51 (8d8 + 15 ) piercing damage.

Burning Chain Scythe (Costs 1 Action). Ranged Weapon Attack: +14 to hit, range 150 ft., one target. Hit: 50 (9d8 + 10) piercing damage and Hit: 51 (10d8+6) fire damage. Has the Returning Property. Mizneria can use one of her attacks on the return of the sickle

Innate Spellcasting. Mizneria casts a spell.

Call of the Lady. (Costs 2 Actions) Mizneria can summon eight fiends (with a challenge rating of 20 or below)

Chilling Gaze. (Costs 1 Action) Mizneria targets one creature within 30 ft., the target makes a Wisdom Save of DC 25 on a failed save target takes 16 (3d10) psychic damage, the target becomes frightened for 30 seconds and gains 1 level of exhaustion. On gaining six levels of exhaustion creature doesn’t die but is dominated by Asmodeus.

Contract Established. (Costs 1 Action) Mizneria on touching a humanoid or a piece of dead humanoid calls its soul back and forms a new adult body for the soul to enter. If the target isn’t willing to do so the spell fails. The body that is formed for the soul is a Tiefling’s body. The soul retains all it’s memories and experiences and is friendly towards Mizneria (it's memories are altered such that it obeys Mizneria).