Arch Hydra

Hold Breath. The hydra can hold its breath for up to 6 hours.

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 30 or more damage in a single turn, one of its heads dies. If all its heads die, the arch 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 fire damage since its last turn. The arch hydra regains 15 hit points for each head regrown in this way.

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

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

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

Bite. Melee Weapon Attack: +12 to hit, reach 15 ft., one target. Hit: 19 (2d10 + 8) piercing damage.

Corruption Breath (Recharge 5-6).  The arch hydra exhales a noxious breath in a 30-ft. cone. Each creature in that area must make a DC 18 Constitution saving throw, taking 54 (12d8) poison damage and become poisoned for 1 minute on a failed save, or half as much damage on a successful one.