Hell Is Dark with No Flowers Vol 5 light novel

Despite winning the competition over Hell, Shiroshi refuses the throne and returns to his old life with Seiji. And soon enough, he has a new mystery on his hands—a family is struck by a series of suspicious deaths, and the husband of one of the victims suspects foul play. But even stranger, he claims the murders are meant to echo the events of "Goldfish," a children's song by Hakushu Kitahara. And in that case, there is sure to be another tragedy...