Hell wasn't won in a day.
Lucifer and the angels who rebelled with him got thrown out of Heaven.
They, along with the inhabitants of Earth, find themselves in a despicable place. They all blame Lucifer for their current predicament and want revenge. Ovnion, Lucifer's old ally and former king of Earth, is the only one to approach Lucifer with a different purpose.
Ovnion wants to ally with Lucifer. He thinks if they work together they might survive. Lucifer will use Ovnion to become the sole ruler over both angels and Earth beings. As the fallen angels and Earth beings fight among themselves, Lucifer will do everything in his power to use their war to his advantage.