Cerberus

Legendary Resistance (3/day). If Cerberus fails a saving throw, it can choose to succeed instead.

Multiple Heads. Cerberus has three heads. While it has more than one head, the Cerberus has advantage on saving throws against being blinded, charmed, deafened, frightened, and stunned.

Whenever Cerberus takes 100 or more damage in a single turn, one of its heads dies. If all its heads die, Cerberus dies. At the end of its turn, any heads that died since its last turn regenerate and Cerberus heals 20 hit points.

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

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.

Multiattack. Cerberus makes as many bite attacks as it has heads.

Bite. Melee Weapon Attack: +13 to hit, reach 10 ft., one target. Hit: 22 (3d8 + 8) piercing damage plus 9 (3d6) necrotic damage.

Hellfire Breath (Recharge 5–6). The hound exhales fire in a 45-foot cone. Each creature in that area must make a DC 20 Dexterity saving throw, taking 24 (6d8) fire damage plus 24 (6d8) necrotic on a failed save, or half as much damage on a successful one.

Cerberus can take 3 legendary actions, choosing from the options below. Only one legendary action can be used at a time and only at the end of another creature's turn. Cerberus regains spent legendary actions at the start of each round. 

Attack. Cerberus makes a Bite attack.

Teleport. Cerberus magically teleports up to 120 ft to an unoccupied space.

Hellfire. Cerberus uses its Hellfire Breath. (2 actions)