Ancient Hydra

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.

Monstrous Regeneration.  Whenever the hydra takes 25 or more damage in a single turn, one of its heads dies. If all its heads die, the hydra is rendered unconscious until it is able to regenerate a head or it falls to 0 hit points and is unable to regenerate further.

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 and gains an additional 10 for each head regrown in this way.

The hydra regains 15 hit points at the start of each turn if it has not taken fire damage since its last turn.

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.

Biological immortality. Any attempt to resurrect an Ancient Hydra works with no material component needed and all rolled healing effects are treated as its maximum possible result.

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

Bite. Melee Weapon Attack: +9 to hit, reach 10 ft., one target. Hit: 12 (1d10 + 6) piercing damage. Roll a DC 15 Constitution Saving throw. On failure take an additional 14 (4d6) poison damage and any rolled healing you receive until the end of your next turn is treated as the lowest possible result.