The Lernaean Hydra, one of the many offspring of half-woman and half-serpent Echidna and 100-headed Typhon, was a many headed serpent who lived in the swamps. One of hydra's heads was impervious to weapons. Its other heads could be cut off, but then one or two would grow back in its place. The breath or venom of the Hydra was deadly. The hydra devoured animals and people in the countryside.
Hercules (Herakles or Hercules) was able to put an end to the depredations of the hydra by having his friend Iolaus cauterize the stump of each head as soon as Hercules cut it off. When only the head impervious to weapons was left, Hercules tore it off and buried it. From the stump, poisonous blood still oozed, so Hercules dipped his arrows in the blood, making them lethal.