Twelve thousand years ago, after the War of Creation, the demigod general Asura assassinated his emperor in an attempt to take the throne. The seven remaining generals retaliated, killing Asura and then taking their place as the Seven Deities, to protect Earth and its people and cleanse the world of demons.
Or so the legends go. In reality, Asura was framed by the other generals, his wife killed, and his daughter kidnapped for her magical abilities. Now that he's escaped from the underworld, you can probably guess that he's a little bit pissed.