Multiattack

The pit fiend makes one Bite attack, one Gore attack, one Mace attack, and one Tail attack, or it makes four Hurl Flame attacks. If two Hurl Flame attacks hit one creature, each creature within 10 feet of the target must make a DC 21 DEX save as the fire bursts outward from the target, taking 14 (4d6) fire damage on a failed save, or half as much damage on a successful one.

Bite

_Melee Weapon Attack:_ +14 to hit, reach 5 ft., one target. _Hit:_ 22 (4d6+8) piercing damage plus 14 (4d6) fire damage and 13 (3d8) poison damage. The target must succeed on a DC 21 CON save or be poisoned. While poisoned in this way, the target can't regain HP. The target can repeat the save at the end of each of its turns, ending the effect on itself on a success.

Gore

_Melee Weapon Attack:_ +14 to hit, reach 10 ft., one target. _Hit:_ 17 (2d8+8) piercing damage plus 14 (4d6) fire damage.

Mace

_Melee Weapon Attack:_ +14 to hit, reach 10 ft., one target. _Hit:_ 15 (2d6+8) bludgeoning damage plus 14 (4d6) fire damage.

Tail

_Melee Weapon Attack:_ +14 to hit, reach 10 ft., one target. _Hit:_ 24 (3d10+8) bludgeoning damage plus 14 (4d6) fire damage, and the target must succeed on a DC 21 STR save or be knocked prone.

Hurl Flame

_Ranged Spell Attack:_ +10 to hit, range 120 ft., one target. _Hit:_ 25 (6d6+4) fire damage.