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 radiant 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. Or if the hydra has three or more heads, you can have it make two bite attacks and the drop head action

Bite. Melee Weapon Attack: +8 to hit, reach 10 ft., one target. Hit: 10 (1d10 + 5) piercing damage. A successful bite attack from one of these heads deals 7 (1d4 + 5) piercing damage, and the target must make a DC 16 Constitution saving throw, taking 7 (2d6) necrotic damage and being cursed with mummy rot on a failed save, or take half as much damage and not be cursed on a successful one.  The cursed target can't regain hit points, and its hit point maximum decreases by 10 (3d6) for every 24 hours that elapse. If the curse reduces the target's hit point maximum to 0, the target dies, and its body turns to dust. The curse lasts until removed by the remove curse spell or other magic.

Split Head  The hydra drops off one of it's soulless heads, which wriggles and jumps around like a Piranha. The hydra loses 10 life and opportunity attacks cannot be made against it until the end of your next turn. Whenever one of its heads dies it drops on the floor and becomes its own creature that acts during the hydra's turn. A severed head has AC 14, 15 hit points, and 5ft of movement. In addition, it has the Hydra's Bite attack, except it cannot curse the target