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 fire 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.

Undead Fortitude. If damage reduces the zombie to 0 hit points, it must make a Constitution saving throw with a DC of 5 + damage taken, unless the damage is radiant or from a critical hit. On a success, the zombie drops to 1 hit point instead.

Undead Nature. The zombie hydra doesn’t require air, food, drink, or sleep.

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.

Poison Breath. Four heads of the zombie hydra exhale poisonous gas in a 60-foot cone. Each creature in the area must make a DC 20 Constitution saving throw, taking 42 (12d6) poison damage on a failed save, or half as much damage on a successful one. A creature that fails this saving throw by 5 or more is poisoned until the end of its next turn.