Asmodeus

Devil's Sight. Magical darkness doesn't impede the devil's darkvision.

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

Regeneration. Asmodeus regains 30 hit points at the start of his turn as long as he hasn't taken any radiant damage

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

Multiattack. The devil makes three melee attacks: two with its fork and one with its tail. It can use Hurl Flame in place of any melee attack.

Fork. Melee Weapon Attack: +10 to hit, reach 10 ft., one target. Hit: 15 (2d8 + 6) piercing damage.

Tail. Melee Weapon Attack: +10 to hit, reach 10 ft., one target. Hit: 10 (1d8 + 6) piercing damage. If the target is a creature other than an undead or a construct, it must succeed on a DC 17 Constitution saving throw or lose 10 (3d6) hit points at the start of each of its turns due to an infernal wound. Each time the devil hits the wounded target with this attack, the damage dealt by the wound increases by 10 (3d6). Any creature can take an action to stanch the wound with a successful DC 12 Wisdom (Medicine) check. The wound also closes if the target receives magical healing.

Hurl Flame. Ranged Spell Attack: +10 to hit, range 150 ft., one target. Hit: 14 (4d6) fire damage. If the target is a flammable object that isn't being worn or carried, it also catches fire.

Ruby Rod. Melee Weapon Attack: +14 to hit, reach 10 ft., one target. Hit: 51 (8d8 + 15 ) piercing damage.

Ruby Rod. Ranged Weapon Attack: +12 to hit, 150/600 ft., one target. Hit: 50 (9d8 + 10) fire damage or 51 (10d8+6) lightning damage.

Horns Melee Attack: +10 to hit, 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 3d10 poison damage. If they succeed, they are no longer poisoned.

Soul Bartering. Asmodeus 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. Asmodeus can make minor changes to the wishes if he wants. Asmodeus suffers no harmful effects from this action.

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

Loyalty to the Lord. As a bonus action, if Asmodeus' rod was ever knocked or taken out of his hand, he can instantly teleport it back into his palm

Corporeal Dodge- When grappled, Asmodeus can turn to ash and smoke for a moment, negating the grapple. 

Ashes to Ashes- When reduced to 0 HP, Asomdeus turns to ashes. If the Ruby Rod is still in his possession, the ashes are instantly teleported to Nesus and sealed in a Folgers Can on top of a pedestal. If the can remains on the pedestal for a long rest, Asmodeus is released and rematerializes in his normal form. The can can not be opened or destroyed. 

Asmodeus 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 Asmodeus regains spent legendary actions at the start of its turn. 

Call of the Hellhounds. (Costs 2 Actions) Asmodeus can summon 3 hellhounds who reside within 9 hells.

Chilling Gaze. Asmodeus target one creature within 30 ft., the target makes a Wisdom Save of DC 25 on a failed save target takes 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. Asmodeus 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 Asmodeus (his memories are altered such that it obeys Asmodeus).