Amodeus

Devil's Sight. Magical darkness doesn't impede Asmodeus' darkvision.

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

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

Immutable Form. Asmodeus is immune to any spell or effect that would alter his form unless he wishes to be affected.

Regeneration. At the start of each of his turns, Asmodeus regains 20 HP. If he takes radiant damage, this trait doesn't function at the start of his next turn.  Asmodeus' body is only destroyed if he starts his turn with 0 HP and doesn't regenerate.  When this occurs on any plane other than the Nine Hells, Asmodeus' body reforms in his citadel Nasheem over the course of 1d10 days.

Hellish Mind. While in the Nine Hells, Asmodeus' mental ability scores increase by 10 each, and his proficiency bonus increases by 1. This adds 6 to all his save DC's and adds 6 to hit with spell attacks.  This increases his challenge rating to 29 out of his lair or 30 in it.

Innate Spellcasting. Asmodeus' spellcasting ability is charisma(Spell Save DC 20).  Asmodeus can innately cast the following spells, requiring no material components.

At will: plane shift, detect magic, hold monster, wall of fire, charm person

Multiattack. Asmodeus makes three melee attacks: two with its pitchfork and one with its tail. He can use Hurl Flame in place of any melee attack.

Pitchfork. Melee Weapon Attack: automatic hit, reach 10 ft., one target. Hit: 17 (2d8 + 8) piercing damage + 60 force damage.  Asmodeus can push the target 5 ft away if it is huge or smaller.

Tail. Melee Weapon Attack: +15 to hit, reach 10 ft., one target. Hit: 12 (1d8 + 8) 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 Asmodeus 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: +12 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.

Blazing Wrath(Recharge 5-6). Asmodeus fills a 60 foot cube with infernal enegy, dealing 45 fire damage to any creatures within it.  Creatures that take any of this damage must make a DC 20 Wisdom saving throw or be stunned till the end of Asmodeus' next turn.

Condemn. Asmodeus targets one or two creatures within 60 ft and forces them to make DC 20 Charisma saves or be sent to the ninth layer of Baator.  Asmodeus may travel with them if he pleases.  If he teleports with them, he must complete a short rest before he can use this ability again.

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

Spell Reflection. If Asmodeus makes a successful saving throw against a spell, or a spell attack misses him, he can choose another creature (including the spellcaster) he can see within 30 feet of him. The spell targets the chosen creature instead of Asmodeus. If the spell forced a saving throw, the chosen creature makes its own save. If the spell was an attack, the attack roll is rerolled against the chosen creature.

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 (Costs 2 Actions).  Asmodeus makes one pitchfork attack.
• Infernal Gaze. Asmodeus targets one creature he can see within 60 feet of him and commands it to combust. If the target can see Asmodeus, it must succed on a DC 26 Wisdom saving throw or take 22(4d10 fire damage).  It if takes any f this damage, it becomes frightened of Asmodeus until the end of his next turn.
• Lightning Storm (Costs 2 Actions). Asmodeus releases lightning in a 20-foot radius.  All other creatures in that area must eah make a DC 26 Dexterity saving throw, taking 18(4d8) lightning damage on a failed save, or half as much damage on a successful one.
• Corrupting Guidance. Asmodeus targets one creature charmed by him that is within 60 feet of him; that charmed target must make a DC 26 Charisma Saving Throw.  On a failed save, Asmodeus decides how that target acts on its next turn.
• Teleport. Asmodeus performs his Teleport action.