The God King is an ancient and immortal being who leads banished eternals (now known as voidwoken) and sworn (beings who sold their souls to him). His main goal is to punish the Seven Gods who served him (as the Seven Lords) and return with his people back to Rivellon.
Aeons before the mortal races walked Rivellon, the God King was a sovereign of Eternals. Under him were the Seven Lords who had betrayed him and banished him into void after Fane told them about his discovery of the Veil. Ever since this event God King has been trying to return to Rivellon and get revenge on the Seven.
In 1242 the God King offers those with a need to attain Source powers in exchange for their physical forms.