Pit Fiend

If the pit fiend 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.

The pit fiend emanates an aura in a 20-foot Emanation [Area Of Effect] while it doesn't have the Incapacitated condition. WIS Save DC 21, any enemy that starts its turn in the aura.
Failure: The target has the Frightened condition until the start of its next turn.
Success: The target is immune to this pit fiend's aura for 24 hours.

If the pit fiend fails a saving throw, it can choose to succeed instead.

The pit fiend has Advantage on saving throws against spells and other magical effects.

The pit fiend makes one Bite attack, two Devilish Claw attacks, and one Fiery Mace attack.

Melee Attack +14, reach 10 ft. 18 (3d6 + 8) Piercing damage. If the target is a creature, it must make the following saving throw. CON Save DC 21.
Failure: The target has the Poisoned condition. While Poisoned, the target can't regain Hit Points and takes 21 (6d6) 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. After 1 minute, it succeeds automatically.

Melee Attack +14, reach 10 ft. 26 (4d8 + 8) Necrotic damage.

Melee Attack +14, reach 10 ft. 22 (4d6 + 8) Force damage plus 21 (6d6) Fire damage.