Hell Hound

Medium fiend, lawful evil

Armor Class 15 (natural armor)
Hit Points 45 (7d8 + 14)
Speed 50 ft.

Skills Perception +5
Damage Immunities fire
Senses darkvision 60 ft., passive Perception 15
Languages understands Infernal but can't speak it
Challenge 3 (700 XP)

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.

Actions

Bite. Melee Weapon Attack: +5 to hit, reach 5 ft., one target. Hit: 7 (1d8 + 3) piercing damage plus 7 (2d6) fire damage.
Fire Breath (Recharge 5–6). The hound exhales fire in a 15-foot cone. Each creature in that area must make a DC 12 Dexterity saving throw, taking 21 (6d6) fire damage on a failed save, or half as much damage on a successful one.