The Lernean hydra was the archetypal original hydra, a serpent-like, multi-headed monster from Greek mythology. It made its lair in the swamps of Lerna in southern Greece and anciently thought to be one of the entrances to the underworld. Popularly, the hydra was said to be able to regenerate any of its heads that might get chopped off, growing back either one or two heads in the lost head's place. One head was immortal and couldn't be killed. Its blood was poisonous.

The offspring of the monsters Typhon and Echidna, the Lernean hydra was raised by the goddess Hera to kill the hero Hercules during the second of his Twelve Labors. Realizing he couldn't slay the hydra by cutting off its heads, Hercules enlisted the help of his nephew Iolaus to cauterize each severed neck with a burning torch before it could regrow it head. For the immortal head, once he chopped it off, Hercules buried it under a very big rock. Afterwards, Hercules dipped his arrows into the hydra's poisonous blood, which he would later use to devastating effect in his adventures.

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Lernean hydra (Wikipedia)