Multiattack

The pit fiend attacks with its bite and mace.

Bite

Melee Weapon Attack: +14 to hit, reach 5 ft., one target. Hit: 22 (4d6 + 8) piercing damage. If the target is a creature it makes a DC 20 Constitution saving throw. On a failure it is poisoned for 1 minute. While poisoned in this way the target can't regain hit points and takes 21 (6d6) ongoing poison damage at the start of each of its turns. The target can repeat this saving throw at the end of each of its turns ending the effect on a success.

Mace

Melee Weapon Attack: +14 to hit, reach 10 ft., one target. Hit: 22 (4d6 + 8) bludgeoning damage plus 21 (6d6) fire damage. If the target is a Medium or smaller creature it makes a DC 22 Strength saving throw. On a failure it is pushed 15 feet away from the pit fiend and knocked prone.

Circle of Fire (1/Day, While Bloodied)

A 15-foot-tall 1-foot-thick 20-foot-diameter ring of fire appears around the pit fiend with the pit fiend at the center. The fire is opaque to every creature except the pit fiend. When the ring of fire appears each creature it intersects makes a DC 18 Dexterity saving throw taking 22 (5d8) fire damage on a failed save or half damage on a successful one. A creature takes 22 (5d8) damage the first time each turn it enters the area or when it ends its turn there. The fire lasts 1 minute or until the pit fiend dismisses it becomes incapacitated or leaves its area.

Fireball (3rd-Level; V, S)

Fire streaks from the pit fiend to a point within 120 feet and explodes in a 20-foot radius spreading around corners. Each creature in the area makes a DC 18 Dexterity saving throw taking 21 (6d6) fire damage on a failed save or half damage on a success.

Hold Monster (5th-Level; V, S, Concentration)

A creature within 60 feet that the pit fiend can see makes a DC 18 Wisdom saving throw. On a failure it is paralyzed for 1 minute. The creature repeats the save at the end of each of its turns ending the effect on a success.