Hydra

The Hydra (Ὕδρα) was the monstrous offspring of Typhon and Echidna. The Hydra lived in the near the spring Amymone, at Lerna. The monster lived with a giant crab.

The Hydra had nine heads. One of them was immortal, and therefore invulnerable to weapons. The other heads were even more dangerous since if one head were cut off, two heads would grow in its place.

Heracles' second labour was to kill the Hydra. To prevent more heads growing from where each head was severed, Heracles' nephew and companion Iolaüs (Iolaus) would use a firebrand to cauterise each neck stump.

Heracles buried the immortal head under a large boulder.