Mouryou no Hako: Chuuzenji Atsuko no Jikenbo - Hako no Yurei no Koto  

Mouryou no Hako - Hako no Yuurei no Koto, The Case Files of Atsuko Chuzenji: 'The Case of the Spirits in the Boxes', Chuzenji Atsuko no Jikenbo 'Hako no Yuurei no Koto', 魍魎の匣～特別編　中禅寺敦子の事件簿「箱の幽霊の事」

A side story that reveals the investigative notes that Atsuko, the younger sister of the main character Akihiko Chuzenji, wrote about the brutal dismembering incidents in the main story.(Source: ANN)