Cerebus, Guardian of the Gates of Hell

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.

The hound makes 3 bite attacks and one tail attack per attack action. Or it can replace each bite attack with an Acid Breath, or Fire Spit 

Bite. Melee Weapon Attack: +11 to hit, reach 15 ft., one target. Hit: 7 (3d8 + 9) piercing damage plus  12 (4d6) acid damage.

Acid Breath (Recharge 5–6). The hound exhales fire in a 15-foot cone from each head . Each creature in that area must make a DC 19 Dexterity saving throw, taking 30 (8d6) Acid  damage on a failed save, or half as much damage on a successful one for each head 

Fire Spit (Recharges when the Acid breath Recharges, meaning it can either use this ability or Acid Breath). (Range 300 ft) The hound spits a concentrated globule of fire that deals damage in a five foot radius. The target must succeed on a DC19 Dexterity saving throw or take (12d6) Fire damage and half as much on a failed save  

Tail whip:  Melee Weapon +8 to hit reach 30ft, The tail wraps around a party member dealing (4d6+5) bludgeoning damage, the target must then succeed on a DC 15 Constitution saving throw, Taking (3d6+4) fire damage on a failed save and have as much on a successful one. Then the target must succeed on DC 15 Strength Save or be grappled. 

The hound has two legendary actions per turn 

The hound can move up to its full movement 

It can charge up to one head regaining acid breath for that head