Asmodeus

Authority of Nessus. Any creature hostile to Asmodeus that starts its turn within line of sight of him must either avert its eyes (if the target is surprised, it cannot avert its eyes) or succeed on a DC 25 Wisdom saving throw at the end of each of its turns. If the target fails three attempts before it succeeds on three attempts, the target is dominated, as in the dominate monster spell. If a creature succeeds on its saving throw of the effect ends for it, it is immune to Authority of Nessus for 24 hours.

Banishment. If Asmodeus is reduced to 0 hit points he explodes in a 40-foot radius ball of flames, and his body is destroyed. However, his essence returns to his stronghold on Nessus and he cannot take physical form for a time. Each creature in the blast area must make a DC 25 dexterity saving throw, taking (12d12) fire damage on a failure and half as much damage on a success.

Innate Spellcasting. Asmodeus' spellcasting ability is Charisma (spell save DC 35, +27 to hit with spell attacks). Asmodeus can innately cast the following spells, requiring no material components, and all are cast at the highest possible level:
At will: [animate dead], [geas], [hold monster], [ice storm], [mass suggestion], [thunderwave], [wall of fire], [wall of ice] 
3/day Each: [blight], [chain lightning], [dominate monster], [force cage], [greater invisibility], [shapechange], [symbol]
1/day Each: [power word heal], [meteor swarm], [trap the soul], [wish]

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

Limited Magic Immunity. Unless Asmodeus wishes to be affected, he is immune to spells of 7th level or lower and has advantage on saving throws of all spells and magical effects.

Magic Weapons. Asmodeus' weapon attacks are treated as +4 magical weapons.

Ruby Rod. The rod is an epic artifact. It has 15 charges, and any of its properties that require a saving throw have a save DC of 35, +27 to hit with spell attacks. While holding it, Asmodeus can use an action to cast [arcane eye], [bestow curse], [fire shield]. Alternatively, he can spend 1 charge to cast one of the following spells from it (at 9th level and double the area/range): [cone of cold], [lightning bolt], [melf's acid arrow]. In addition, when he hits a creature with a melee attack with the rod he can choose to immediately spend 2 charges to cast [dominate monster] on the target.

Multiattack.  Asmodeus makes two Slam or two Ruby Rod attacks.

Ruby Rod. Melee Weapon Attack: +26 to hit, reach 10 ft., 1 target. Hit: 61 (9d8 + 2) bludgeoning damage plus 36 (8d8) cold, fire, or acid damage, Asmodeus' choice.

Slam. Melee Weapon Attack: +25 to hit, reach 5 ft., 1 target. Hit: 50 (9d6 + 19) bludgeoning damage plus 21 (8d6) fire damage and teh target must succeed on DC 32 Strength saving throw or be knocked prone or grappled, Asmodeus' choice.

Hellfire Pillar (recharge 5-6). Asmodeus chooses a point on the ground that he can see within 480 feet of him. A cylinder of hellfire, 100 feet tall with a 20-foot radius, rises from that point and lasts for 1 minute. Creatures in the area when the pillar erupts, or that end their turn in the area, lose their immunity or resistance to fire damage, or gain vulnerability to fire damage if they did not have any resistance or immunity, for 1-minute. In addition, each creature must make a DC 35 Dexterity saving throw, taking (20d20) fire damage on a failure, or half as much damage on a success.

Fling.  (Bonus Action) One medium or smaller object or creature held or grappled by Asmodeus is thrown up to 300 ft. in a direction of its choosing and knocked prone. If a thrown target strikes a solid surface, the target takes (1d6) bludgeoning damage for every 10 feet that it was thrown. If the target is thrown at another creature, that creature must succeed on a DC 32 Dexterity saving throw or take the same damage and be knocked prone.

Puppet Master. (Bonus Action) Asmodeus exerts precise control over one creature he has dominated.

Summon Devil (2/short rest). (Bonus Action) Asmodeus magically summons any one devil of his choosing to fight for him.

Hellfire Rebuke (1/day).  When Asmodeus is the target of a ranged weapon or spell attack, Hellfire Pillar recharges and he can use a reaction to use it immediately.

Asmodeus can take 4 Legendary Actions, choosing from the options below. Only one legendary action option can be used at a time and only before or after another creature's turn. If Asmodeus has unspent legendary actions at the end of the round, he may spend and immediately use his remaining legendary actions. Asmodeus regains spent legendary actions at the start of his turn.

Attack. Asmodeus can make a ruby rod or slam attack.
Move. Asmodeus can move up to half his speed.
Regenerate. Asmodeus regains 50 hit points.
Ruby Rod (1/round). Asmodeus can use the ruby rod to cast a spell or spend one of its charges.
Innate Spellcasting (Cost 2 Actions). Asmodeus casts a spell.
Chill Gaze (Costs 2 Actions). Asmodeus can force one creature within 240 feet of him that can see him to look into his eyes. The creature must make a DC 35 Wisdom saving throw, taking (10d12) cold damage and be paralyzed on a failure, and half as much damage on a successful saving throw.
Summons (Costs 2 Actions). Asmodeus can use his summon devils ability.
Teleport (Costs 2 Actions). Asmodeus magically teleports, along with any equipment he is carrying, up to 480 feet to an unoccupied space he can see.
Recharge (Costs 3 Actions). Asmodeus recharges hellfire pillar.