A long time ago the Gods and Goddesses fought against Hades and his servants(devils). It took great power to defeat Hades so the only place they had the ability to send him to was earth, to be reborn as a human. Before the Gods and Goddesses fall into a deep sleep (due to overusing power), they left behind their crystals that contained the last of their power so that the chosen humans can protect their own kind from the devils, whom now roams freely on Earth without their ruler.