Hydra of Lerna more often known simply as the Hydra, was aserpentine water monster in Greek and Roman mythology. Its lair was the lake of Lerna in the Argolid, which was also the site of the myth of the Danaids. Lerna was reputed to be an entrance to the Underworld and archaeology has established it as a sacred site older than Mycenaean Argos. In the canonical Hydra myth, the monster is killed by Heracles, using sword and fire, as the second of his Twelve Labors.The Hydra was the offspring of Typhon and Echidna. It possessed many heads,for every head that was cut down it grew two more. Also one of its heads was inmortal.Additionaly,the Hydra had poisonous breath and blood so virulent that even its scent was deadly.
The Hydra guards the entrance to the Underworld and from the murky swamps of the Lake of Lerna the monstrous serpent would rise and terrorize the city. The Hydra was finally killed by Hercules during his second labor.