Hold Breath. The Worm Hydra can hold its breath for 1 hour.

Multiple Heads. The Worm Hydra has 8 - 10 mouths. While it has more than one mouth, the Worm Hydra has advantage on saving throws against being charmed, frightened, stunned, and knocked unconscious.

Whenever the Worm Hydra takes 25 or more damage in a single turn, one of its mouths dies. If all its mouths die, the Worm Hydra dies.

At the end of its turn, it grows two mouths for each of its mouths that died since its last turn, unless it has taken fire damage since its last turn. The Worm Hydra regains 10 hit points for each mouth regrown in this way.

Wakeful. While the Worm Hydra sleeps, at least one of its mouths is awake.

Multiattack. The Worm Hydra makes as many bite attacks as it has mouths.

Bite. Melee Weapon Attack: +8 to hit, reach 10 ft., one target. Hit: 10 (1d10 + 5) piercing damage.

on a natural 18 +, make a grapple check.  If target is grappled they automatically take bite damage each round on the Worm Hydras turn.  On the targets turn they can attempt to escape.

Reactive Heads. For each mouth the Worm Hydra has beyond one, it gets an extra reaction that can be used only for opportunity attacks.