It was the second of Heracles' twelve labours to kill the hydra.
During the fight, everytime Heracles chopped off one of the hydra's nine heads, two more grew in its place. So Heracles asked for the help of his nephew Iolaus, who would cauterize each stump when Heracles chopped off a head, preventing the regrowth of two heads. Because the last of the heads was immortal, Heracles was forced to bury it under a large boulder.
After killing the Hydra, Heracles dipped his arrows in the blood of the beast. These arrows were used to kill Geryon. However, this labour was not counted as Heracles was aided by Iolaus.
The breath of the hydra is so poisonous that it could kill a man just by inhalation. The foul stench that the river of Elis, Anigrus, exudes is the smell of the decaying Hydra.
Also in Greek mythology, Jason killed another hydra to get the Golden Fleece.