Cerberus, King of Hellhounds

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

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

Magic Weapons. All of cerberus’ melee attacks are considered as magical.

Legendary Resistance (1/day). If Cerberus fails a saving throw, it can choose to succeed instead.

Pack Tactics. Cerberus 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.

King of Hellhounds. Any hellhounds that can see cerberus or lives inside cerberus territory must follow the cerberus' orders over those of any masters, and over any free will the hellhound might have. 

Multiple Heads. Cerberus has three heads, and therefore has advantage on saving throws against being deafened, stunned, or knocked unconscious. 

Multiattack. Cerberus makes three bite attacks, one with each head, and one claw attack, or foregoes one bite attack to make a breath weapon attack with that head. 

Bite. Melee weapon attack: +11 to hit, reach 5 ft., one target. Hit: 20 (3d8 + 6) piercing damage plus 14 (4d6) fire, cold, or necrotic damage, depending on which head is attacking. 

Claw. Melee weapon attack: +11 to hit, reach 10 ft., one target. Hit: 23 (3d10 + 6) slashing damage. 

Fire Breath (Recharge 5–6). Cerberus exhales fire in a 30-foot cone. Each creature in that area must make a DC 17 Dexterity saving throw, taking 42 (12d6) fire damage on a failed save, or half as much damage on a successful one. 

Cold Breath (Recharge 5-6). Cerberus exhales cold in a 30-foot cone. Each creature in that area must make a DC 17 Constitution saving throw, taking 44 (8d10) cold damage on a failed save, or half as much damage on a successful one. 

Necrotic Breath (Recharge 5-6). Cerberus exhales necrotic energy in a 30-foot cone. Each creature in that area must make a DC 17 Constitution saving throw, taking 39 (6d12) necrotic damage 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 its turn.

Claw. Cerberus makes a claw attack. 

Abyssal Recharge (Costs 2 Actions). Cerberus recharges an expended breath weapon of its choice. 

Bites (Costs 3 Actions). Cerberus makes two bite attacks with two seperate heads of its choice. 

Converging breaths (Costs 3 Actions). If none of Cerberus' breath weapons require recharge, it may expend all three breath weapons at once to combine them into a singular, devastating attack. Cerberus exhales fire, cold, and necrosis in a 90-foot cone. Each creature in that area must succeed a DC 18 Constitution saving throw or take 32 (9d6) fire damage, plus 33 (6d10) cold damage, plus 33 (5d12) necrotic damage. If a target succeeds their saving throw, they take no damage, but the immense stress excerted upon their body leads to them gaining 2 points of exhaustion.