In Greek mythology, the Lernaean Hydra (Greek: was an ancient nameless serpent-like chthonic water beast that possessed numerous heads— the poets mention more heads than the vase-painters could paint— and poisonous breath (Hyginus, 30). The Hydra of Lerna was killed by Heracles as one of his Twelve Labours. Its lair was the lake of Lerna in the Argolid, though archaeology has borne out the myth that the sacred site was older even than the Mycenaean city of Argos, for Lerna was the site of the myth of the Danaids. Beneath the waters was an entrance to the Underworld, and the Hydra was its guardian (Kerenyi 1959, p. 143).
The details of the confrontation are explicit in Apollodorus (2.5.2): realising that he could not defeat the Hydra in this way, Hercules called on his nephew Iolaus for help. His nephew then came upon the idea (possibly inspired by Athena) of using a burning firebrand to scorch the neck stumps after decapitation, and handed him the blazing brand. Hercules cut off each head and Iolaus burned the open stump leaving the Hydra dead; its one immortal head Hercules placed under a great rock on the sacred way between Lerna and Elaius (Kerenyi1959 p 144), and dipped his arrows in the Hydra's poisonous blood, and so his second task was complete. The alternative to this is that after cutting off one head he dipped his sword in it and used its venom to burn each head so it couldn't grow back.
Hercules later used an arrow dipped in the Hydra's poisonous blood to kill the centaur Nessus; and Nessus's tainted blood was applied to the Tunic of Nessus, by which the centaur had his posthumous revenge. Both Strabo and Pausanias report that the stench of the river Anigrus in Elis, making all the fish of the river inedible, was reputed to be due to the Hydra's poison, washed from the arrows Hercules used on the centaur.
When Eurystheus, the agent of ancient Hera who was assigning to Hercules The Twelve Labours, found out that it was Hercules' nephew who had handed him the firebrand, he declared that the labour had not been completed alone and as a result did not count towards the ten labours set for him. The mythic element is an equivocating attempt to resolve the submerged conflict between an ancient ten Labours and a more recent twelve.