Diabolical Restoration. If Yacxuthl'zhush dies outside the Nine Hells, its body disappears in sulfurous smoke, and it gains a new body instantly, reviving with all its Hit Points somewhere in the Nine Hells.

Annihilation Aura. Yacxuthl'zhush emanates an aura in a 20-foot Emanation while it doesn’t have the Incapacitated condition. Constitution Saving Throw: DC 25, any enemy that starts its turn in the aura. Failure: The target takes 10d12 necrotic damage and is frightened and knocked prone. Success: The target takes 2d12 necrotic damage.

Legendary Resistance (4/Day). If Yacxuthl'zhush fails a saving throw, it can choose to succeed instead.

Magic Resistance. Yacxuthl'zhush has Advantage on saving throws against spells and other magical effects. Yacxuthl'zhush is immune to any spells with a level below 7.

Multiattack. The pit fiend makes one Destroying Bite attack, two Devilish Claw attacks, and one Eldritch Blade attack.

Destroying Bite. Melee Attack Roll: +20, reach 20 ft. Hit: (6d12 + 30) Piercing damage. If the target is a creature, it must make the following saving throw. Constitution Saving Throw: DC 25. Failure: The target has the Poisoned condition. While Poisoned, the target can’t regain Hit Points and takes (10d6) Poison damage at the start of each of its turns, and it repeats the save at the end of each of its turns, ending the effect on itself on a success.

Devilish Claw. Melee Attack Roll: +20, reach 25 ft. Hit: (8d8 + 30) Necrotic damage.

Eldritch Blade. Melee Attack Roll: +20, reach 50 ft. Hit: (10d12 + 30) Force damage plus 21 (6d6) Fire damage.

Hellfire Spellcasting (Recharge 4–6). Yacxuthl'zhush casts Fireball (level 9 version) up to 3 times, requiring no Material components and using Charisma as the spellcasting ability (spell save DC 25). It can replace one Fireball with Hold Monster (level 9 version) or Wall of Fire (level 9 version).