Armoured Undead Pit Fiend

Death Aura. The pit fiend is surrounded in a 15ft aura of fog, any creature that ends their turn in the fog must succeed a DC 17 constitution saving throw or take 20 (3d8) necrotic damage and have their maximum hit points reduced by their constitution modifier until the end of their next turn.

Magic Resistance. The pit fiend has advantage on saving throws against 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 21). The pit fiend can innately cast the following spells, requiring no material components:

At will: fear, fireball,animate dead

3/day each: hold monster, wall of fire

1/day each: finger of death, circle of death

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

Bite. Melee Weapon Attack: +14 to hit, reach 5 ft., one target. Hit: 22 (4d6 + 8) 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. On hit, target must succeed DC 17 constitution saving throw or take an extra 8 (2d6) necrotic 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.

Teleport. The pit fiend magically teleports, with anything its wearing or carrying, up to 120ft to a point it can see.

3/daily

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