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

Fiendish Regeneration. Mephistopheles regains 20 hit points at the start of his turn. If he takes radiant damage this trait doesn't function at the start of his next turn. Mephistopheles only dies if he starts his turn with 0 hit points and is unable to regenerate.

Flyby. Mephistopheles doesn't provoke opportunity attacks when he flies out of an enemy's reach.

Hellfire Mastery. Fire damage that Mephistopheles inflicts ignores resistances and immunities. Mephistopheles is immune to this effect.

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

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

Spellcasting. Mephistopheles casts one of the following spells, requiring no material components and using Intelligence as the spellcasting ability (spell save DC 26):

At will: Darkness, Detect Evil and Good, Detect Magic, Dispel Magic, Geas (duration 1 year), Greater Restoration, Hallow, Hold Monster, Locate Creature, Locate Object, Major Image, Resurrection, Scrying, Suggestion, Wall of Fire

1/day each: Meteor Swarm, Symbol, Wish

Multiattack. Mephistopheles makes three Ranseur attacks. He can replace one of the attacks with Hellfire Lash (if available).

Ranseur +3. Melee Weapon Attack: +17 to hit, reach 10 ft., one target. Hit:20 (2d10 + 9) piercing damage plus 18 (4d8) fire or cold damage (Mephistopheles' choice).

Hellfire Implosion (Recharge 4–6). A bright streak of Hellfire appears at a point that Mephistopheles can see within 150 feet of him. Each creature in a 20-foot-radius sphere centered on that point must make a DC 22 Dexterity saving throw. Targets take 31 (9d6) fire damage on a failed save, or half as much damage on a successful one.

Hellfire Lash (Recharge 5–6). Mephistopheles unleashes a 60-foot-long, 5-foot-wide lash of Hellfire that ignites a 5-foot- radius sphere around where it strikes. Any targets in the area of effect must make a DC 22 Dexterity saving throw, taking 36 (8d8) fire damage on a failed save, or half as much on a successful one.

Ashen Teleport. Mephistopheles' body and any equipment he is wearing or carrying turns to ash, then he magically teleports up to 120 feet to an unoccupied space he can see and reforms.

Hellfire Wings. Mephistopheles uses his wings to generate a burst of heat. Each creature within 10 feet of him must succeed on a DC 22 Dexterity saving throw or take 9 (2d8) bludgeoning damage plus 9 (2d8) fire damage. Creatures that failed the save also have the prone condition.

Hellfire Storm (Costs 2 Actions). Mephistopheles creates a rain of Hellfire at a point he can see within 150 feet. Each creature within a 20-foot-radius sphere centered on that point must make a DC 24 Dexterity saving throw. Targets take 38 (7d10) fire damage on a failed save, or half as much damage on a successful one.

Call Underling (Costs 3 Actions). Mephistopheles summons an allied horned devil in an unoccupied space that he can see.