Akuma o Awaremu Uta

Hokkaido Prefectural Police detective Ryohei Akutsu is investigating the "Box-Folding Serial Murders" that occurred eight years ago. Despite the bewildered looks of those around him, he diligently pursues the case, and during this time he meets throat physician Akihisa Shikane. A quiet and capable doctor, Shikane promises to cooperate. Meanwhile, Akutsu is tormented by internal conflicts within the Hokkaido Prefectural Police...and Shikane is, in fact, the "Box-Folding Killer" himself!! An apocalyptic crime suspense story colored with blood, violence, and memories begins here!!

(Source: Shinchosha, machine translated)

Notes:
-The series was canceled in 2019 and was originally collected in four volumes. In 2024, the author released a fifth volume to conclude the series.
-The chapter count includes three extras.