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. When a head dies this way it can make a DC 25 Constitution saving throw, it succeeds it remains allive on the ground only being able to take reactions can be used only for opportunity attacks.

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.

Undead Fortitude. If damage reduces the zombie to 0 hit points, 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 drops to 1 hit point instead. 

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.  The target must make a 16 con save or take 3d6 poison dmg

The Hydra can take 1 legendary action, choosing from the options below. Only one legendary action option can be used at a time and only at the end of another creature's turn. The Hyndra regains spent legendary actions at the start of its turn.

Projectile Vomiting (1 action). The Hydra vomits acid in a 30-foot line that is 5 feet wide. Each creature in that line must make a DC 14 Dexterity saving throw, taking 20 (5d8) acid damage on a failed save and must make a DC 14 Constitution saving throw or be poisoned until the end of their next turn, or half as much damage on a successful one.
When the Hydra takes that action it looses on of its heads.