In Greek mythology, the Hydra was a giant water snake with many heads that lived in a swamp near Lerna in the land of Argos. The number of heads is variously reported from as few as 5 to more than 100.
immortal able to live forever
The second of the 12 labors of Hercules* was to kill the Hydra. However, when one of the Hydra's heads was cut off, two more grew in its place. The monster also had one immortal head. To defeat the Hydra, Hercules called on his friend Iolaus for help. As soon as Hercules cut off one head, Iolaus would seal the wound with a hot iron or a torch so that nothing could grow to replace it. After removing the Hydra's immortal head, Hercules buried it under a large rock. He then collected the monster's poisonous blood. In later adventures, he dipped his arrows in the blood so that they would instantly kill whomever they struck.