Bloodwar Combatant. The Hellhound has advantage on all attacks against fiends.

Rage Against Devils. When the Hellhound starts its turn within 30 feet of a fiend, it enters a frenzied state. The Hound must use its movement to get as close to the nearest fiend as it is able to, and must use its action to attack the nearest fiend.

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

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

Bite. Melee Weapon Attack: +5 to hit, reach 5 ft., one target. Hit: 1d6 + 3 piercing damage plus 1d6 fire damage.

1/Day: Fire Breath . The hound exhales fire in a 15-foot cone. Each creature in that area must make a DC 12 Dexterity saving throw, taking 4d6 fire damage on a failed save, or half as much damage on a successful one.