The hydra is the fifth-level creature of the Dungeon in Heroes of Might and Magic V. With its high hit points, it's likely to survive combat, and the Three-headed Attack, Enraged and No enemy retaliation abilities (which lets it attack all creatures in front of it, gives it additional attack for every friendly stack that dies, and stops the enemy from retaliating, respectively) ensures that it will do some serious damage. The Hydra can be upgraded to the deep hydra (or, in Tribes of the East, the foul hydra).