The Labors of Herakles (known as Hercules to the Romans)
The goddess Hera, determined to make trouble for Herakles, made him lose his mind. In a confused and angry state, he killed his own wife and children. 
When he awakened from his "temporary insanity," Herakles was shocked and upset by what he'd done. He prayed to the god Apollo for guidance, and the god's oracle told him he would have to serve Eurystheus, the king of Tiryns and Mycenae, for twelve years, to atone for the murders. Herakles had to perform 12 labors, feats so difficult that they seemed impossible. 

Labor Two: The Story of Herakles and the Hydra

As part of his second labor, King Eurystheus sent Herakles to seek out and destroy the monstrous, nine-headed Hydra. The Hydra lived in the swamps of Lerna, and anyone who tried to kill the creature by cutting off a head, would find that two more heads would grow in its place.  In addition, the blood of the Hydra was poisonous, and its very breath lethal. Herakles sought out the monster in its lair and brought it out into the open with flaming arrows. The Hydra encircled its many heads around Herakles and called on an ally, a huge crab that also lived in the swamp. The crab bit at Herakles’s heel to slow down his attack. Herakles called on his nephew, Iolaos, who had driven Herakles to Lerna in a chariot. Responding to his uncle’s call, Iolaos grabbed a burning torch and as Herakles cut off one of the Hydra's heads, Iolaus was there to sear the wounded neck with the flame. This kept further heads from growing from the wound. Herakles cut off the heads one by one, with Iolaus searing the wounds. Finally Herakles lopped off the one head that was supposedly immortal and buried it deep beneath a rock.