A rebellion happened sometime before (or after) the Creation of the World that separated the Forces of Light from the Forces of Chaos. The cause of this is open for debate: some say that the leader of the rebellion, Shmjzh, refused to honor the creation of man; some others believed that he wanted to gain supremacy equal to that of the Supreme One. Whatever the case, one third of the Forces of Light joined behind his cause and were cast out of Heaven.
The Forces of Light defend the world and keep it from going into utter chaos. Sometimes, however, the Legions of Hell are able to get past them and they cause chaos by doing so. Fortunately, the Forces of Light are ensured victory in the end.