The forbidden soul-summoning book "Song of Chant", which has been sealed away by the Black Moon Order for generations since the Middle Ages, has been stolen. Mysterious black magic and more than a hundred kinds of battle summons have flowed into the world, and the originally stable and peaceful continent has entered a period of turmoil. The giant alien creatures developed by humans to resist unknown summoning spells have gone out of control due to immature technology. More and more "monsters" appear in different sea areas, forests or wilderness. When military weapons became increasingly unable to deal with the activity and destructive power of mutated creatures, people began to try to borrow the summoning skills of the Black Moon Order to deal with these giant monsters. As time evolved, a true demon-hunting corps, the "Royal Deacons", a global demon-contractor mercenary organization supported by the Sanside royal family and established by Archibald, the former second bishop of the Black Moon Island Order, followed the trend of the times and stood at the pinnacle of the job of killing alien creatures.
