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Huge monstrosity, unaligned
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.
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.
Bite. Melee Weapon Attack: +8 to hit, reach 10 ft., one target. Hit: 10 (1d10 + 5) piercing damage.
The hydra is a reptilian horror with a crocodilian body and multiple heads on long, serpentine necks. Although its heads can be severed, the hydra magically regrows them in short order. A typical specimen has five heads.
Everlasting Hunger. A rapacious and gluttonous monster, a hydra snatches and tears its prey in a frenzy of feeding. When a hydra has cleared a territory of food and driven off any creatures smart enough to avoid it, it moves on to seek its meals elsewhere. A hydra's hunger is so great that if it can't feed, it might turn against itself, its heads attacking each other as the creature eats itself alive.
Hardy Water Dwellers. Hydras are natural swimmers, dwelling in rivers, along lakeshores, in ocean shallows, and in wetland bogs. A hydra rarely requires shelter from the elements, so it doesn't normally have a lair. Only in colder climes are hydras drawn to the protection of sheltered caverns and ruins.