Ursidean Hydra

Acidic Blood. When the hydra takes piercing or slashing damage, each creature within 5 feet of the hydra takes 5 (1d10) acid damage.

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.

Whenever the hydra takes 30 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, plus one additional claw attack.

Bite. Melee Weapon Attack: +9 to hit, reach 10 ft., one target. Hit: 9 (2d6 + 6) piercing damage.

Claw. Melee Weapon Attack: +8 to hit, reach 5 ft., one target. Hit: 12 (2d10 + 4) slashing damage.

Absorb Bears. If any type of bear or bear-adjacent creature is reduced to 0 hit points while within 10 feet of the hydra, it can use the reaction of one of its heads to consume the bear. When the hydra does this, it instantly sprouts another head, regaining 10 hit points, and forcibly bursting a new head out, triggering its Acidic Blood trait. The hydra can additionally use this reaction if it moves within reach of a bear corpse within 1 minute of the bear dying.