Eurystheus commanded Heracles to perform twelve labors in twelve years in order to keep him away from his kingdom, afraid Heracles would take his throne. Eurystheus ordered Heracles to kill the Lernaean hydra. This hydra, raised in the swamp of Lerna, would go out into the plain and destroy both property and livestock. The hydra is a gigantic multi-headed dragon, with anywhere from 6 to 9 heads depending on which version of the story you read with the middle one being immortal. It's breath and blood were deadly.