Her Demon: Leviathan

Before humanity, many people believed in the existence of angels and demons. The Almighty created angels to protect all living things. Peace and harmony did not last long, however, because of the deeds of one specific angel who called himself Lucifer and desired rebellion. The terrestrial and cosmic revolution was the name given to it. They recognize the fallen angels there and then. One of the fallen angels seemed to have been given a second chance, but he could only change his fate if the mission were successful, the only opportunity to enter paradise.