Eris is the goddess of chaos and uses the body of Elien, who is a friend of Hyoga, to revive herself. She obtains the golden apple to drain Athena's life energy to make her ressurection complete and to be able to turn the world in a place filled with chaos. But to be able to attack Eris, Seiya and his friends will first have to defeat the Ghost Knights.

(Source: Anime News Network)