Hellhound

• Keen Hearing and Smell. The Hell Hound has advantage on Wisdom (Perception) checks that rely on hearing or smell.
• Pack Tactics. The Hell Hound has advantage on an attack roll against a creature if at least one of the hound's allies is within 5 ft. of the creature and the ally isn't incapacitated.
• Three-Headed. The Hell Hound has advantage on Wisdom (Perception) checks and on saving throws against being blinded, charmed, deafened, frightened, stunned, and knocked unconscious.

Actions

• Multiattack. The Hell Hound makes three bite attacks, one with each head.
• Bite. Melee Weapon Attack: +7 to hit, reach 5 ft., one target. Hit: 7 (1d8 + 3) piercing damage plus 7 (2d6) fire damage.
• Fire Breath (Recharge 5-6). The three-headed Hell Hound exhales fire. Each creature in a 15-foot cone must make a DC 15 Dexterity saving throw. A creature takes 63 (18d6) fire damage on a failed save, or half as much on a successful one.
  
  A single-headed Hell Hound exhales fire. Each creature in a 15-foot cone must make a DC 15 Dexterity saving throw. A creature takes 21 (6d6) fire damage on a failed save, or half as much damage on a successful one.
• Call the Pack. Only a Three-Headed Hell Hound can split from one being into three as an Action. The two copies are partly shrouded in smoke but are otherwise identical to the original. Each version acts like a One-Headed Hell Hound. All three share a single health pool. If one of the copies takes more than a third of the max HP of the Hellhound in a single attack, it is immediately banished and returns back to the original. The Hound can recall its copies as a bonus action.