The Lernaean Hydra was a monster in Greek mythology. It had many heads and every time someone would cut off one of them, two more heads would grow out of the stump. It was one of the offspring of Typhon and Echidna, the father and mother of all monsters respectively. It lived in the lake Lerna in the region of Argolid in the Peloponnese.
Eurystheus, king of Tiryns, sent the demigod Heracles to slay the Lernaean Hydra as the second of the set of labours that the hero had to complete, in order to redeem himself for killing his wife Megara and his children in a fit of madness. When Heracles reached the lake Lerna, he had to cover his nose and mouth with a cloth, in order to protect himself from poisonous gases that were emitted. After he managed to lure the monster out of its lair, Heracles quickly cut one of its heads off, only to realise in despair that two new heads had grown.
So, he decided to ask for the help of his nephew Iolaus, who thought of using fire to cauterise the stump as soon as the head is cut off. So, they made a plan; Heracles would cut off the heads and Iolaus would burn the wound with a firebrand. As a result, they both quickly started winning the battle. Hera, who had raised the monster specifically to kill Heracles, sent a giant crab in aid of the hydra. The hero, though, crushed it under his giant foot. The final head of the Hydra was immortal, and Heracles managed to destroy it by using a golden sword that the goddess Athena had offered him. Then, he dipped all of his arrows into the venomous blood of the hydra. This would later be the hero's demise, as the Shirt of Nessus which Heracles unknowingly wore was covered in the tainted blood, causing him unbearable pain.