Corrupted Hydra

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

Multiple Heads. The hydra has six 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 fire 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. It can use a Flame Jet or Frost Jet in place of a bite attack.

Bite. Melee Weapon Attack: +8 to hit, reach 10 ft., one target. Hit: 10 (1d10 + 5) piercing damage.

Flame Jet (Recharge 5-6 per Head). The hydra exhales flame in a 20-foot line that is 10 feet wide. Each creature in that line must make a Dexterity saving throw, taking 10 (3d6) fire damage on a failed save, or half as much damage on a successful one.

Frost Jet (Recharge 5-6 per Head). The hydra exhales frost in a 20-foot line that is 10 feet wide. Each creature in that line must make a Constitution saving throw, taking 10 (3d6) cold damage on a failed save, or half as much damage on a successful one.

The save DC for the Flame Jet and Frost Jet is equal to 13 + the hydra's proficiency bonus.