If the heads are cut off, the heads would grow back. One head cut-off would result to two heads growing back in its place.
The Hydra was believed to have lived in the Lernean marsh which is located near Argolis, the region around Argos, Greece. Others say that the Hydra lived in Cave in the Swamp of Lerna
The serpent-woman Echidna and the hundred headed Typhon are the Hydra’s parents. His siblings include the Nemean lion, Cerberus, Chimera and Ladon.
The Hydra guards the entrance to the Underworld and from the murky swamps of the Lake of Lerna the monstrous serpent would rise and terrorize the city. The Hydra was finally killed by Hercules during his second labor.
The Hydra was said to have the body of a dragon/snake with many heads (possibly 7, 8 or 9), two arms & legs with knife-like claws, sharp spines/spikes & a long serpent tail.
Killing the Hydra: The Second Labor of Hercules
Accompanied with his trusty nephew, Iolaus, Hercules set off to hunt the nine-headed monster. They went to the springs of Amymone and discovered the lair of the menacing beast.
Hercules lured the creature out of its den by shooting it with flaming arrows. When the beastly creature emerged, the Greek mythical hero seized it but the monster wound one of its coils to Hercules’ foot.
With one of his foot stuck, Hercules tried to break free by smashing the monster’s head, but as soon as he cut one, two more heads would appear on its place. And a huge crab began biting Hercules’ trapped foot to add nuisance. After smashing the crab with his club, Hercules called on to his nephew, Iolaus to help him out in fighting the looming monster.
Hercules persisted on slashing the monster’s head while Iolaus scorched each headless neck with a torch to prevent heads from growing back. Finally, the Hydra was slain as Heracles's second task was done.
After the Hydra was defeated, Heracles soaked the tip of his arrows in its venomous blood.