The Lernaean Hydra, also known as the Hydra of Lerna or more often simply known as the Hydra, is a gigantic serpent with nine heads, with the center one being immortal. It is one of the spawn of Typhon and Echidna.
The each head that was cut off, two more grew. The monster’s haunt was the marshes of Lerna near Argos. The Lernaean Hydra was killed by Heracles for the second of his Twelve Labors. There were also other Hydra, called Hydrae, that had some physical differences.
In media, a hydra was treated as a sub-species of a dragon.
Eight Hydras guard the Underground Lake in the Valley of Despair which houses the Flood Tide Orb. The jewel is required to extinguish the flames surrounding Belial's Castle, and can only be retrieved by Undine.