The labour of the Lernaean Hydra was Heracles' second mission and challenge
The Lernaean Hydra was a water beast with nine snake-like heads and poisonous breath that was living in lake Lerna of Argos. The Lernean Hydra was the offspring of Typhon and Echidna and the sister of the Lion Nemea, which Heracles had feated on his first labour.
With the help of his nephew, Iolaos, Heracles found the monster in the source Amimoni. Using an axe he began to cut its heads, but on every attempt, two new ones would always come out in their place.
Heracles then told Iolaos to turn on a torch and to burn the flesh immediately, after having cut off the monster's head. The idea this was crowned with success and no new ones could grow out.
When he managed to also cut off its ninth head, which was in the middle of body and immortal, Heracles buried it deeply into the ground and put a huge stone on it. Then he dunked his arrows in the bile of Hydra, which was poisonous.
King Evrystheas, however, declared the labor as incomplete with the excuse that Iolaos had helped Heracles.