The God Of All Things once ruled the universe. Her loyal servants were both the humans and the Angels but she favored the humans more than the Angels so the Angels lived in her home (names Heaven by the humans) and obeyed her every whim. Those Angels more exhalted carried occasional messages to the humans but most were confined to Heaven.
One day the right-hand of the God Of All Things, who had been gaining power and followers for many centuries rose up and challenged the God, tired of seeing his people going un-rewarded for their hard work while the humans did nothing and were given highest honors. This Angeli's name was Lucifer. Locked in a bloody battle, the Angels of Heaven were divided in half by the power struggle. Half followed the God, half followed Lucifer. Both the God and Lucifer sent Angeli and Angeli into battle for centuries until finally, to end the war, the God and Lucifer met each other one-on-one.
Ultimately, the God was defeated but before Lucifer could strike his finishing blow the God did something completely unexpected. Instead of using her last bit of power to save herself she closed the gates of Heaven, sealing its doors to Angels and humans alike. With no home anymore and drunk on their victory the Angels, turned and invaded Aearth, taking revenge on the humans who had been their former master's pets and who had refused to side in the war.
The Angels united under Lucifer and did battle with the human, who were weak with under preparation for battle. Years past and the human's freedom fell. Generations past and the Angels were given free reign over Aearth under the rule of Lucifer. The humans? Every last one became slaves to the Angels.