Book Marker(侵入者; Bukkumāka) also known as Intruder, is a Sacred Gear with a rather interesting twist: While other sacred gears are hosted by only one user per generation, this exclusive one can manifest itself upon several users at the same time. This Sacred Gear has the ability to insert it's user into a certain event in a certain timeflow and thus allowing it to change history. First introduced in DxD: Haremonogatari, this particular Sacred Gear is discussed even among the characters from Highschool DxD: Яe-birth, sealed away by the God Indra, deeming 'uverused' by him.