Ancient Infernal Hydra

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

Multiple Heads. The hydra has five heads. While it has more than one head, the hydra has advantage on saving throws against being blinded, charmed, deafened, frightened, stunned, and knocked unconscious.

Whenever the hydra takes 35 or more damage in a single turn, one of its heads dies. If all its heads die, the hydra dies.

At the end of its turn, it grows two heads for each of its heads that died since its last turn, unless it has taken cold damage since its last turn. The hydra regains 10 hit points for each head regrown in this way.

Regeneration: The hydra regains 5 hit points at the start of its turn for each head it has unless it has taken cold damage since its last turn. The hydra dies only if it starts its turn with 0 hit points and no heads.

Reactive Heads. For each head the hydra has beyond one, it gets an extra reaction that can be used only for opportunity attacks.

Wakeful. While the hydra sleeps, at least one of its heads is awake.

Multiattack. The hydra makes as many bite attacks as it has heads.

Bite. Melee Weapon Attack: +8 to hit, reach 10 ft., one target. Hit: 10 (1d10 + 5) piercing damage and 10 (1d10 + 5) fire damage.

Tail Swipe: Melee Weapon Attack: +8 to hit, reach 10 ft., one target. Hit: 10 (1d10 + 5) bludgeoning damage.

Fire Breath (1/day): When the hydra has 3 or fewer heads, each head can use a fire breath attack. Each head breathes fire in a 15-foot cone. Each creature in that area must make a DC 17 Dexterity saving throw, taking 7 (2d6) fire damage per head on a failed save, or half as much damage on a successful one.