Acidic Body Fluids. When the zombie hydra takes piercing or slashing damage, each creature within 5 feet of the zombie hydra takes 9 (2d8) acid damage.

Multiple Heads. The zombie hydra has five heads. While it has more than one head, the zombie hydra has advantage on saving throws against being blinded, charmed, deafened, frightened, stunned, or knocked unconscious. Whenever the zombie 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 Zombie hydra regains 10 hit points for each head regrown in this way.

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

Undead Fortitude. If damage reduces the zombie hydra to 0 hit points or causes its last head to die, it must make a Constitution saving throw with a DC of 5 + the damage taken, unless the damage is radiant or from a critical hit. On a success, the zombie hydra regrows two heads and regains 10 hit points for each head regrown in this way.

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

Viral Aura. Any creature that starts its turn within 10 feet of the zombie hydra must make a DC 17 Constitution saving throw. On a failed save, the creature is poisoned for a minute and can’t regain hit points until the end of its next turn. On a successful save, the creature is immune to this zombie hydra’s Viral Aura for 24 hours.

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

Bite. Melee Weapon Attack: +10 to hit, reach 15 ft., one target. Hit: 17 (2d10 + 6) piercing damage plus 4 (1d8) acid damage.

Virulent Miasma (1/Day). The zombie hydra releases toxic gas in a 30-foot-radius sphere centered on itself. Each creature in that area must make a DC 17 Constitution saving throw, taking 14 (4d6) poison damage on a failed save, or half as much damage on a successful one. A Humanoid reduced to 0 hit points by this damage dies and rises as a zombie 1 minute later. The zombie acts immediately after the zombie hydra in the initiative count.