Hell Is Dark with No Flowers, Vol. 6 (light novel)

When a novelist dies and leaves behind a mysterious memo reading "the black cat has returned to life," Shiroshi and Seiji head to his estate to investigate the truth. There, at an eerie building that was once a hospital, they find the relatives of the deceased...along with the Rindou brothers. As tension rises between the rival detectives and among the remaining family members, the night brings a string of deaths corpses torn apart by some wild beast in the dark. Could this be the "black cat" from the author's note?