Asmodeus

Magic Weapons. Asmodeus's weapon attacks are magical. He has the following item on his person: The ruby rod of Asmodeus which he can use all features of even though they are not listed on the stat block. 

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

Discorporation. When Asmodeus drops to 0 hit points or dies, his body is destroyed but his essence travels back to his domain in Nessus, and he is unable to take physical form for a time.

Limited Magic Immunity. Unless he wishes to be affected, Asmodeus is immune to spells of 4th level or lower. He has advantage on saving throws against all other spells and magical effects.

Regeneration. Asmodeus regains 20 hit points at the start of his turn. If he takes damage from a +3 weapon or item, this trait doesn’t function at the start of his next turn. Asmodeus dies only if he starts his turn with 0 hit points and doesn’t regenerate.

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

At will: Animate dead, Blasphemous flame*, Charm monster, Create undead (9th level), Detect good and evil, Detect magic, Dispel magic (7th level), Dominate monster, Forsaken blood (9th level)*, Geas, Greater restoration, Hellflame blast*, Impish agitator*, Major image, Mass suggestion (8th level), Project image, Ruby rule*, Suggestion

3/day each: Blight (8th level), True resurrection, Power word stun, Wall of ice

1/day each: Meteor swarm, Power word heal, Power word kill, Power word pain, Symbol Wish

Spellcasting. Asmodeus is a 20th-level spellcaster. His spellcasting ability is Wisdom (spell save DC 27, +19 to hit with spell attacks). Asmodeus has following Cleric spells prepared:
Cantrips (at will): Guidance, Mending, Resistance, Sacred flame, Spare the dying
1st level (4 slots): Cure wounds, Detect poison and disease, Sanctuary, Shield of faith
2nd level (3 slots): Hold person, Lesser restoration, Prayer of healing, Spiritual weapon
3rd level (3 slots): Glyph of warding, Protection from energy, Remove curse, Speak with dead
4th level (3 slots): Banishment, Divination, Freedom of movement, Locate creature
5th level (3 slots): Dispel evil and good, Flame strike,  Mass cure wounds, Planar binding
6th level (2 slots): Blade barrier, Harm, Heal 
7th level (2 slots): Divine word, Etherealness, Fire storm
8th level (1 slots): Antimagic field, Earthquake
9th level (1 slots): Gate, Mass heal
* Means the spell is within Hellish depths pdf.

Multi attack. Asmodeus makes three attacks with his ruby rod and casts a spell a long with using one of his gaze actions. 

Ruby rod of Asmodeus. Melee Weapon Attack: +23 to hit, reach 5 ft., 1 target. Hit: 32 (4d8 + 14) Bludgeoning damage plus 16 (3d10) necrotic damage.

Chilling gaze. Asmodeus gazes at a creature who can see his eyes within 30ft of himself causing them to make a DC 27 Wisdom saving throw. On a failed saving throw the creature is under the effects of the slow spell for 1 minute. A target can remake the saving throw at the end of their next turn. 

Crippling fear gaze. Asmodeus gazes at a creature who can see his eyes within 60ft of himself causing them to make a DC 27 Wisdom saving throw. On a failed saving throw the creature is frightened for 1 minute and has disadvantage on all strength based attack rolls, skill checks and saving throws. A target can remake the saving throw at the end of their next turn. 

Teleport. Asmodeus magically teleports, along with any equipment he is wearing and carrying, up to 120 feet to an unoccupied space he can see.

Summon Devil. Asmodeus chooses what to summon and attempts a magical summoning.

Asmodeus has a 100 percent chance of summoning 1 pit fiend or 2 of any other type of greater or lesser devil. 

A summoned devil appears in an unoccupied space within 60 feet of Asmodeus, acts as an ally of Asmodeus, and can’t summon other devils. It remains for 1 minute, until it or Asmodeus dies, or until Asmodeus dismisses it as an action.

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.

Teleport (Costs 1 Action). Mephistopheles uses his Teleport action. 

Cast a spell (Costs 1 Action). Asmodeus casts a spell from their their Innate spellcasting or Spellcasting trait. 

Lord of the Nine (Costs 3 Actions). Asmodeus lets forth an eruption of infernal power which effects all non devils within 120ft of himself. Each creature within range must make a DC 27 Constitution saving throw or take 94 (9d20) fire damage which bypasses all resistances. On a successful saving throw a target takes half damage. To use this legendary action Asmodeus must sacrifice 15 hit points.