A Letter at the Beginning: Strangeness Is Coming

High school student Gao Lin unexpectedly received a letter that read: "When you see this letter, it means something strange has arrived;" "Ghosts are everywhere, be careful of the people around you, even the people closest to you may be ghosts;" "Remember, ghosts cannot be killed by humans, only ghosts can kill ghosts; If you want to survive, you must either become a ghost, or contract with a ghost and become a ghost controller! "