Undying Devil

Unbeatable. If you kill the devil you will start to hear voices in your head that tell you to put on his crown. putting on the crown will turn you into the devil by corrupting and killing you. you can escape this fate with a successful DC 24 Perception check.

Mimicry. The Undying Devil can mimic any sounds it has heard, including voices. A Creature that hears the sounds can tell they are imitations with a successful DC 16 Insight check.

Double Axe. Melee Weapon Attack: +4 to hit, reach 5 ft., one target. Hit: 10 (2d6 + 2) slashing damage.

Multiattack. The Undying Devil attacks two times on a turn. 

Slam. Melee Weapon Attack: +6 to hit, reach 5 ft., one target. Hit: 10 (2d4 + 2) Bludgeoning damage.

Otherworldly Perception. The Devil can sense the presence of any creature within 50 feet of it that is invisible or on the Ethereal Plane. It can pinpoint such a creature that is moving.

Distrupt Life (Costs 3 Actions). Each non-undead creature within 30 feet of the Devil must make a DC 17 Constitution Saving throw against this magic, taking 18 (3d6) necrotic damage on a failed save or half as much damage on a successful one.