Cryohydra

Hold Breath. The cryohydra can hold its breath for 1 hour.

Ice Walk. The cryohydra can move across icy surfaces without needing to make an ability check. Additionally, difficult terrain composed of ice or snow doesn’t cost it extra movement.

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, or knocked unconscious.

Whenever the cryohydra takes 40 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.

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

Wakeful. While the cryohydra sleeps, at least one of its heads is awake.

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

Bite. Melee Weapon Attack: +10 to hit, reach 15 ft., one target. Hit: 11 (1d10 + 6) piercing damage plus 5 (1d10) cold damage.

Frost Breath (Recharge 5 - 6). The cryohydra exhales a plume of icy wind in a 30-foot cone. Each creature in that area must make a DC 18 Constitution saving throw. On a failure, they take 11 (2d10) cold damage for each head the cryohydra has, or half as much damage on a successful save.