Arzothon the Betrayed, Arch-Demon Prince

Innate Spellcasting. Arzothon’s innate spellcasting ability is Charisma (spell save DC 20, +13 to hit with spell attacks). He can innately cast the following spells, requiring no material components:

Cantrips (at will): fire bolt, light, mage hand, prestidigitation, shocking grasp

1st level (4 slots): detect magic, identify, mage armor, magic missile, hellish rebuke, shield

2nd level (4 slots): detect thoughts, mirror image, misty step, shatter, scorching ray, darkness

3rd level (4 slots): counterspell, lightning bolt, vampiric touch, thunder step, melf's minute meteors, hunger of hadar, fireball, erupting earth, dispel magic

4th level (3 slots): banishment, fire shield, stoneskin, wall of fire, vitriolic sphere, shadow of moil, raulothim's psychic lance, greater invisibility, giant insect, evard's black tentacles, blight

5th level (3 slots): cone of cold, scrying, wall of force, negative energy flood, mislead, maelstrom, insect plague, immolation, far step*, enervation, dominate person, destructive wave, danse macabre, contagion, cloudkill

6th level (3 slots): globe of invulnerability, summon fiend, soul cage, scatter, programmed illusion*, otto's irresistible dance, mass suggestion, flesh to stone, chain lightning

7th level (2 slots): teleport, reverse gravity, resurrection, power word pain, plane shift, fire storm

8th level (2 slots): mind blank*, power word stun, maddening darkness, earthquake, dominate monster, antimagic field

9th level (1 slot): weird

*Arzothon casts these spells on himself before combat.

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

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

Magic Weapons. Arzothon's weapon attacks are magical.

Magical Mind. Arzothon can hold concentration on three spells at once.

Regeneration. Arzothon 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. Arzothon dies only if he starts his turn with 0 hit points and doesn’t regenerate.

Multiattack. Arzothon makes two attacks, either two with his sword, or one with his sword and any other attack.

Flame Wreathed Flamberge. Melee Weapon Attack: +13 to hit, reach 15 ft., hits any creature in 15 ft. semi-circle in front of Arzothon. Hit: 16 (2d6 + 7) slashing damage plus 13 (2d8) fire damage.

Grasp of Death. Melee Weapon Attack: +13 to hit, reach 15 ft., one target. Target must roll DC 20 CON saving throw, success results in half damage. Hit: (5d6) necrotic damage.

Ray of Fire. Melee Weapon Attack: +13 to hit, reach 120 ft. in straight line from Arzothon, targets within 5 ft. of line will be hit. Targets must roll DC 20 DEX saving throw, success results in half damage. Hit: (3d6) fire damage.

Fall through the Fire (Recharge 5–6). Arzothon gazes into the eyes of one creature. The target must succeed on a DC 22 Wisdom saving throw or be frightened until the start of the creature’s next turn and take (5d8) psychic damage, or half as much on a success, as they see the Nine Hells and all of the layers of the Abyss flash in their mind through Arzothon's eyes.

Arzothon can take 3 legendary actions, choosing from the options below. Only one legendary action option can be used at a time and only at the end of another creature’s turn. Arzothon regains spent legendary actions at the start of his turn.

Attack. Arzothon attacks once with his Curved Great Sword or Ray of Fire.

Chaos Incarnate (Costs 2 actions). Arzothon causes chaotic magic to well up in a spellcaster he can see, the target must succeed on the DC 22 Wisdom saving throw or roll once on the wild magic surge table.

Shadow Step. Arzothon teleports to a location he can see within 60 ft.

Summon Fiend (Costs 2 actions). Arzothon summons one CR 4 or lower fiend in an unoccupied space that he can see.