Hold Breath. The cryohydra can hold its breath for 1 hour.
Multiple Heads. The cryohydra has five heads. While it has more than one head, the cryohydra 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 cryohydra 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 fire damage since its last turn. The cryohydra regains 10 hit points for each head regrown in this way.
Wakeful. While the cryohydra sleeps, at least one of its heads is awake.
Ice Walk. The cryohydra can move across and climb icy surfaces without needing to make an ability check. Additionally, difficult terrain composed of ice or snow doesn't cost it extra movement.
Cold Hide. When a creature hits the cryohydra with a melee attack, the creature takes 6 cold damage.
Multiattack. The cryohydra 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 plus 5 (2d8) cold damage.
Cold Breath (Recharge 5–6). Each of the hydra's heads exhales frigid mist and shards of ice in a 30-foot line that is 10 feet wide. Each one of these lines originates in a different square adjacent to the hydra, and no line can pass through a space occupied by the hydra.
Each creature in a line must make a DC 16 Dexterity saving throw, taking 18 (4d8) cold damage on a failed save, or half as much damage on a successful one. Creatures affected by the breath of multiple heads must make a saving through for each line they're caught in.
Reactive Heads. For each head the cryohydra has beyond one, it gets an extra reaction that can be used only for opportunity attacks.