Corrupted 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 25 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 acid damage since its last turn. The hydra regains 10 hit points for each head regrown in this way.

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 (1d4 + 2)

The corrupted hydra pulses sporadically with deep purple energy that crawls its way up to a random head. 

Corrupted Poison Breath (Recharge 5–6). The corrupted hydra pulses sporadically with deep purple energy that crawls its way up to a random head that exhales poisonous gas in a 30-foot cone. Each creature in that area must make a DC 14 Constitution saving throw, taking 42 (12d6) poison damage on a failed save, or half as much damage on a successful one.

Corrupted Thrash. If the hydra loses one of its heads, its corruption pulses with a painful jolt to the hydra causing it to thrash uncontrollably. All creatures within 10ft of the hydra make a DC 12 Dexterity saving throw taking 12 (4d6) bludgeoning damage.