A hydra can be killed by severing all of its heads or slaying its body. Any attack that is not an attempt to sever a head affects the body, including area attacks or attacks that cause piercing or bludgeoning damage. To sever a head, an opponent must make a sunder attempt with a slashing weapon targeting a head. A head is considered a separate weapon with hardness 0 and hit points equal to the hydra's HD. To sever a head, an opponent must inflict enough damage to reduce the head's hit points to 0 or less. Severing a head deals damage to the hydra's body equal to the hydra's current HD. A hydra can't attack with a severed head, but takes no other penalties.