In Greek mythology, Hydra Lernaia, more often simply known as Hydra, was described as a gigantic and mighty nine-headed water-serpent, who ravaged the marshes around the lake Lerna in the region of Argolid in the Peloponnese, said to be one of the entrances to the Underworld. Like Cerberus, the multi-headed terrible dog vigilantly guarding the banks of the River Styx in the Underworld, Hydra was also one of the offsprings of Typhon, the personification of volcanic forces and Echidna, the mother of all monsters.
The Hydra was formidable and almost invincible with its nine heads and every time someone would cut off one of them, two more heads would grow out of the stump. To make matters even worse, its middle head was immortal and could never be killed. In addition to that, its blood and venom were poisonous, even its breath was also deadly.
It is therefore obvious that like the other giant monsters of Greek mythology, Hydra Lernaia had no match in the mortal world and only a superhero could even hope to defeat the creature. However, at the end, Heracles, the illegitimate son of Zeus, killed the invincible Hydra by his ruthless bronze sword, gifted by the goddess Athena.
Hera, the wife of Zeus, hated Heracles or Hercules and wanted to destroy him, as she was aware that Heracles was the illegitimate son of her husband and a mortal woman named Alcmene. By applying her dirty tricks, Hera applied a spell on Heracles that drove him to temporary insanity and induced him to murder his beloved wife and their two children.
However, when he came to his senses, he repented earnestly and begged to Apollo, the god of truth and healing, to be punished for his criminal sin. Despite being aware of Hera’s vengeful trick, Apollo advised Heracles to suffer the indignities to work twelve years under his cousin, King Eurystheus of Mycenae and complete ten impossible tasks to expiate his sins, described in mythology as the twelve labours of Heracles.
At the insistence of Hera, Eurystheus sent Heracles to destroy the gigantic nine-headed water-serpent, as one of his twelve labours. To complete this mission, Heracles took the help of his nephew Iolaus, to use fire to cauterize the stumps as soon as the heads of the monster are cut off, to prevent them to grow again. As they reached the lake Lerna, they had to cover their noses and mouths, in order to protect themselves from the poisonous breaths of the deadly monster.
After that, as the monster came out of its lair, Heracles started to sever each of its mortal heads one by one, while Iolaus successfully cauterized the fresh wounds with a torch to prevent them growing back. Sensing the imminent crisis of Hydra, who was raised specifically to kill Heracles, Hera sent a giant crab in its aid, which Heracles crushed under his powerful feet. Finally, he severed the immortal head of the monster by using a golden sword, gifted by goddess Athena and buried the head under a heavy rock, still alive and writhing, to complete his task.
According to the Greek and Roman writers, after Heracles slew the Hydra and the crab, Hera placed the Hydra high above in the sky as the constellation of the same name, while the crab turned into constellation Cancer.