From the introduction:
You are members of an organization that fights against an endless plague of monsters. Every night, a new apocalypse is unleashed on the world, and demons, ghouls, devils, fallen angels, and other twisted creatures boil up from hell. After centuries of fighting, most civilizations have fallen. Few havens are left. It is up to you to choose how you fight. Emulate the sacred dove and wield the fragmented knowledge of diving miracles, or become the demon and tear your enemies apart. Whatever your choice, seek out Old Hunter Bruan for your next mission.