Welfineus

Keen Hearing and Smell. The wolf has advantage on Wisdom (Perception) checks that rely on hearing or smell.

Pack Tactics. The wolf has advantage on an attack roll against a creature if at least one of the wolf's allies is within 5 feet of the creature and the ally isn't incapacitated.

Shapechanger. Welfineus can use its action to polymorph into a direwolf. All stats become that of a wolf. If Welfineus dies in it's wolf form, he changes back to his real form and is fully healed. If this happens, he can't use this ability for 48 houres.

Magic Resistance. The balor has advantage on saving throws against spells and other magical effects

Fear Aura. Any creature hostile to the pit fiend that starts its turn within 20 feet of Welfineus must make a DC 21 Wisdom saving throw, unless Welfineus is incapacitated. On a failed save, the creature is frightened until the start of its next turn. If a creature's saving throw is successful, the creature is immune to Welfineus's Fear Aura for the next 24 hours.

Multiattack. Welfineus makes three attacks: one with its bite and two with its claws.

Bite. Melee Weapon Attack: +14 to hit, reach 10 ft., one target. Hit: 19 (2d10 + 8) piercing damage plus 7 (2d6) fire damage.

Claw. Melee Weapon Attack: +14 to hit, reach 10 ft., one target. Hit: 17 (2d8 + 8) slashing 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.

Fire Breath (Recharge 5–6). The dragon exhales fire in a 60-foot cone. Each creature in that area must make a DC 21 Dexterity saving throw, taking 63 (18d6) fire damage on a failed save, or half as much damage on a successful one.