Ath'tazeth

Fear Aura. Any creature hostile to the pit fiend that starts its turn within 20 feet of the pit fiend must make a DC 25 Wisdom saving throw, unless the pit fiend is incapacitated. On a failed save, the creature is frightened until the start of its next turn. If a creature's saving throw is successful, the creature is immune to the pit fiend's Fear Aura for the next 24 hours.

Magic Immunity. The pit fiend is immune to spells and other magical effects.

Magic Weapons. The pit fiend's weapon attacks are magical.

Innate Spellcasting. The pit fiend's spellcasting ability is Charisma (spell save DC 25). The pit fiend can innately cast the following spells, requiring no material components:

At will: fireball

10/day each: wall of fire

Multiattack. The pit fiend makes eight attacks: two with its bite, two with its claw, two with its mace, and two with its tail.

Bite. Melee Weapon Attack: +14 to hit, reach 5 ft., one target. Hit: ? (6d6 + 12) piercing damage. The target must succeed on a DC 21 Constitution saving throw or become poisoned. While poisoned in this way, the target can't regain hit points, and it takes 21 (6d6) poison damage at the start of each of its turns. The poisoned target can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success.

Claw. Melee Weapon Attack: +14 to hit, reach 10 ft., one target. Hit: 17 (2d8 + 8) slashing damage.

Mace. Melee Weapon Attack: +14 to hit, reach 10 ft., one target. Hit: 15 (2d6 + 8) bludgeoning damage plus 21 (6d6) fire damage.

Tail. Melee Weapon Attack: +14 to hit, reach 10 ft., one target. Hit: 24 (3d10 + 8) bludgeoning damage.