Mouryou no Hako: Chuuzenji Atsuko no Jikenbo - Hako no Yurei no Koto
Japanese title: 魍魎の匣～特別編　中禅寺敦子の事件簿「箱の幽霊の事」
Type: Special
Episodes: 1
Source: Novel
Genres: Mystery, Supernatural, Suspense
Studios: Madhouse
Duration: 16 min
Rating: R - 17+ (violence & profanity)

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)