Asmodeus, Lord of Nessus

Shapechanger. Asmodeus can use his action to polymorph into a form that resembles a medium humanoid, another large humanoid form, or his true form. Aside from his size, his statistics are the same in each humanoid form, but when he shifts to his true form, those statistics apply (See Asmodeus, The Raging Fiend).

Spellcasting. Asmodeus is a 5th-level spellcaster. His spellcasting ability is Wisdom (spell save DC 18, +8 to hit with spell attacks). Asmodeus has following Sorcerer spells prepared:
Cantrips (at will): Acid Splash, Fire Bolt, Ray of Frost, Shocking Grasp, Mage Hand
1st level (5 slots): Charm Person, Chromatic Orb, Shield, False Life, Thunderwave
2nd level (4 slots): Darkness, Detect Thoughts, Levitate, Scorching Ray, Shatter
3rd level (4 slots): Counterspell, Fireball, Fly, Slow
4th level (3 slots): Banishment, Stoneskin, Wall of Fire
5th level (1 slot): Cone of Cold, Dominate Person

Legendary Resistance (4/Day). If Asmodeus fails a saving throw, he can choose to succeed instead.

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

Magic Weapons. Any weapon attacks made my Asmodeus are considered magical.

Multiattack. Asmodeus attacks twice with his claws, and can use his Frightful Presence ability.

Claws. Melee Weapon Attack: +10 to hit, reach 10 ft., one target. Hit: 28 (4d8 + 12) slashing damage. 

Frightful Presence. Each creature of Asmodeus’ choice within 120 feet of him and aware of him must succeed on a DC 16 Wisdom saving throw or become frightened for 1 minute. A frightened creature can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success. These later saves have disadvantage if Asmodeus is within line of sight of the creature.

If a creature succeeds on any of these saves or the effect ends on it, the creature is immune to Asmodeus' Frightful Presence for the next 24 hours.

Form Shift. If Asmodeus is reduced to 0 hit points, a scream of agony followed by a dark shroud consumes the Lord of Nessus, and the resulting fog, mist and cloud all but blinds everything in a 200ft radius. After one round, Asmodeus emerges in his true form. (See Asmodeus, The Raging Fiend)

Asmodeus can take 3 legendary actions, choosing from the options below. Only one legendary action 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. 

Attack. Asmodeus makes one Claw attack.
Silver Tongue. Asmodeus can choose to cast Charm Person on one creature within range. This consumes a spell slot.
Rend (Costs 2 Actions). Asmodeus can slice through a target with ferocity that will cause the target to continually bleed. Make a Claw Attack against one target. On a hit, in addition to the base damage, the target must succeed on a DC 18 Constitution saving throw or be left with an open wound. The creature will take an additional 3d10 damage at the start of each of it's turns unless the wound is staunched, healed, or removed by another effect. A creature with a successful DC 18 Wisdom (Medicine) check can stop the bleeding, or use a Healer's Kit to do so. This requires the creature to be adjacent to the affected target and consumes one action.