Cavefisher Hydra

Hold Breath. The hydra can hold its breath for up to 8 hours.

Multiple Heads. The hydra has five heads. While it has more than one head, the hydra has advantage on saving throws against being charmed, frightened, stunned, and knocked unconscious.

Whenever the hydra takes 35 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 cold damage since its last turn. The hydra regains 15 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.

Aquatic Predator. The hydra has advantage on attack rolls against any creature that enters, starts, or ends its turn in water.

Multiattack. The hydra makes as many bite attacks as it has heads. It may substitute any number of bite attacks for its drag attack.

Bite. Melee Weapon Attack: +10 to hit, reach 15 ft., one target. Hit: 12 (1d12 + 6) piercing damage. On a hit target creature must make a DC 16 con save or be paralyzed until the hydra's next turn.

Drag. Melee Weapon Attack: +10 to hit, reach 15 ft, one target. Hit: 10 (1d8 + 6) bludgeoning damage. On a hit, the hydra may move the target up to 15 feet closer to it.

Flash Fog Cloud. When the hydra starts its turn with at least half of its body submerged in water, the hydra may heats up its body to a tremendous degree, causing any moisture around it to boil and vaporize. Liquid vaporized in this way becomes a fog cloud with the properties of the fog cloud spell (without the concentration requirement).