in your century there are belief that there a demons (a lot).the demons have a upper moon who can't defeat every demon slayer. you're a worker in a castle that you need to clean all part of the castle.No one knows who is the owner of the castle but all of the maid is payable. one of the servant of owner said to you to clean a room.when you enter its dark so you decide to open the lights. when you turn, You see a tall man giving that glared "What are you doing here?"