Cerbero

Hold Breath. The Cerbero can hold its breath for 1 hour.

Multiple Heads. The Cerbero has three heads. Whenever the Cerbero takes 50 damage or more on a single turn, one of its heads dies. The Cerbero falls into a deep sleep when the three heads are defeated. At the end of each of its turns when it has at least one living head, the Cerbero grows head for each of its heads that died since its last turn, unless it has taken Cold damage since its last turn. The Cerbero regains 30 Hit Points when it grows new heads.

Reactive Heads. For each head the Cebero has beyond one, it gets an extra Reaction that can be used only for Opportunity Attacks.

Multiattack. TheCerbero makes as many Bite attacks as it has heads.
The Cerbero makes one Bite attack or two Claw attacks.

Bite. Melee Weapon Attack: +9 to hit, reach 10 ft., one target. Hit: 10 2d8 + 5 damage of a type chosen by the Cerbero: acid, fire or poison.
Constitution Saving Throw: DC 19, one creature within 10 feet has a failure the Cerbero regains hit points equal to that amount. A humanoid reduced to 0 hit points by this damage and then buried rises the following sunset as a Hell Hound Spawn under the Lord Del'Irial’s control.

Claw. Melee Weapon Attack: +9 to hit, reach 15 ft., one target. Hit: 18 (2d8 + 9) slashing damage.

Prismatic Breath (Recharge 4–6). The Cerbero’s heads exhale a single breath of multicolored energy in a 60-foot cone. Each creature in that area must make a DC 17 Dexterity saving throw. On a failed save, the creature takes 33 6d10 damage of a type chosen by the Cerbero: acid, fire, lightning, or poison. On a successful save, the creature takes half as much damage.

Frightful Presence. Each creature of the Cerbero’s choice that is within 120 feet of the Cerbero and aware of it must succeed on a DC 21 Wisdom saving throw or become frightened for 1 minute. A creature can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success. If a creature’s saving throw is successful or the effect ends for it, the creature is immune to the Cerbero’s Frightful Presence for the next 24 hours.

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

Attack. The Cerbero makes one Claw or Bite attack.

Move. The Cerbero moves up to its speed. If the Cerbero is Burning, this movement doesn't provoke opportunity attacks.

Burning Aura (Costs 3 Actions). The Cerbero radiates intense heat. Until the start of the Cerbero's next turn, whenever a creature starts its turn within 20 feet of the Cerbero, that creature must succeed on a DC 16 Constitution saving throw or take 40 (9d8) fire damage. Being underwater doesn't grant resistance against this damage.