Satan

One day a big rift appeared. Out came legions of monsters and demons, as well as Satan, the ruler of hell, himself. He is spreading suffering through the world and he is recruiting people he deems worthy to become his direct servant demons. Through a prophecy you know that in 2 years the troops of heaven will come to defend earth against the troops of heaven. Untill then you have to hold out and somehow prevent Satan from destroying the whole world.