Asmodeus

Legendary Resistance (5/Day). If Asmodeus fails a saving throw, He can choose to succeed instead.

Regeneration. If Asmodeus has at least 1 hit point, He regains 50 hit points at the start of His turn.

Limited Magic Immunity. Asmodeus is immune to spells of 5th level or lower, only affecting Him if He chooses them to. He has advantage against all other spells and magical effects.

Innate Spellcasting. Asmodeus spellcasting ability is Charisma (spell save DC 27, +19 to hit). He can innately cast the following spells, requiring no material components:

At will: fire bolt, chill touch, bestow curse, fireball, counterspell, geas, chain lightning, wall of fire

1st Level (4 Slots): disguise self, identify
2nd Level (3 slots: detect thoughts, blindness/deafness,  enthrall, hold person, phantasmal force 
3rd Level (3 Slots): dispel magic, slow
4th Level (3 slots): banishment, phantasmal killer
5th Level (3 slots): dominate person, hold monster
6th Level (2 slots): mental prison, harm
7th Level (2 slots): finger of death, forcecage, divine word 
8th Level (2 slots): feeblemind, dominate monster
9th Level (2 slots): imprisonment, meteor swarm

Fear Aura. Any creature hostile to Asmodeus that starts its turn within 60 feet of Him must make a DC 27 Wisdom saving throw, unless Asmodeus is incapacitated. On a failed save, the creature is frightened until the start of its next turn. If a creature's saving throw is successful, the creature is immune to the Fear Aura for the next 24 hours.

The Ruby Rod of Asmodeus. As evil as it’s creator, the rod is none other than the Lord of Nessus’ personal implement of pain. The rod functions as a magic mace that grants a +3 bonus to attack and damage rolls made with it and, as a bonus action, you can cause the head of the rod to erupt in flame dealing an additional 3d6 fire damage per hit. The rod also has the following properties;

• Drain Life. When you hit a creature with a melee attack using the rod, you can force the target to make a DC 20 Constitution saving throw. On a failure, the target takes an extra 8d6 necrotic damage, and you regain a number of hit points equal to that necrotic damage. You can use this ability once per dawn.
• Paralyze. When you hit a creature with a melee attack using the rod, you can force the target to make a DC 20 Strength saving throw. On a failure, the target is paralyzed for 1 minute. The target can repeat the saving throw at the end of each of its turns, ending the effect on a success. You can use this ability once per dawn.

Spells. The Ruby Rod has 9 charges. You can use an action to expend 1 or more charges to cast one of the following spells (Save DC20) from it: bane (1 charge), command (1 charge), hex (1 charge), divine word (4 charges), finger of death (4 charges), plane shift (4 charges), power word pain (4 charges), power word kill (5 charges)

Master Enchanter. When you cast a spell from the Enchantment school of magic, you can twin cast the spell, targeting a second creature within range.

Soul Trap. When a creature dies within 60 feet of Asmodeus while He holds the rod, He can use a reaction and expend 1 charge to snatch the soul of the dying. He traps the soul inside the rod, as in the spell soul cage, but with the following changes. A stolen soul remains inside the rod until the spell ends or if the Ruby Rod is destroyed. While the soul is trapped it can be exploited in any of the ways described below. You can use a trapped soul up to six times. Once the soul has been used for the sixth time, it is released and the spell ends. While a soul is trapped, the dead form it came from can not be revived by any means beyond a wish. The rod can only contain one soul at a time.
Drain Life. Using a bonus action you can drain the life force from the soul, regaining 2d8 hit points.
Infernal Fuel. You can use trapped souls to power the abilities of the Ruby Rod. Each exploitation of a soul counts as one charge, up to the max of 6 charges, where upon the soul is released. 
Query Soul. You ask the soul a question (no action required) and receive a brief telepathic answer. The soul knows what it knew in life and must answer truthfully to the best of its ability. The answer is no more than a sentence or two and can be as cryptic or straightforward as desired.
Mark of the Beast. You imprint upon the soul a mark of infernal runes that scars it. Upon its release, should it be returned to its vessel, its body is now cursed. While cursed in this way the creature has horrid visions of torture and the nightmarish landscape of the Hell's. They can not gain the benefits of a long rest, as the nightmares haunt their waking and resting hours, and any hit points that would be healed by magic are halved. If they die while cursed in this way their soul is sent to the River Styx and they emerge along its shores as a lemure. A greater restoration, remove curse or similar spell ends the curse on the target. 

Multiattack. Asmodeus makes three attacks, two with the Ruby Rod and one with His tail.

Ruby Rod. Melee Weapon Attack: +21 to hit, reach 10 ft., One target. Hit: 15 (2d6 + 8) plus an additional 10 (3d6) fire damage. In addition He uses one of the effects of the Ruby Rod on one of the strikes. 

Tail. Melee Weapon Attack: +18 to hit, reach 10 ft., one target. Hit: 25 (3d10 + 8) bludgeoning damage.

The King of Hell is Legendary in every sense of the word. He can take 5 legendary actions per turn 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. Asmodeus regains spent legendary actions at the start of His turn.

Cantrip. Asmodeus uses a cantrip.

Tail. Asmodeus makes one tail attack.

Teleport (Costs 2 Actions). Asmodeus magically disappears in a pillar of flame, all creatures within 20 feet of Him must succeed on a DC 24 Dexterity save or suffer 32 (8d8) fire damage and He reappears in an unoccupied space He can see within 120 feet.

Hellfire (Costs 3 Actions). Asmodeus targets up to 5 targets He can see within 60 feet unleashing the flames of Hell upon them. Each target must succeed against a DC 24 Dexterity saving throw or suffer 63 (16d6) fire damage. The flames stick to creatures who fail causing them to lose resistance to fire damage, or turn immunity to resistance, until the end of their next turn. On a success you take half as much fire damage and resistances are unaffected. 

Summon Legion (Costs 3 Actions). Asmodeus commands the Legion to come to His aid. The King of Hell calls for 1 Pit Fiend, 1d4 Horned Devil or 2d6 Spined Devil to come appear within 100 feet of him. While in the Nine Hell's there is no chance for error. Beyond the Nine Hell's, roll a d20 and on a 1 the summoning falters resulting in the arrival of 2d6 Lemure.