Hell Is Dark with No Flowers, Vol. 2 (light novel) - Yoru Michio - Bog

A letter hinting at an impending murder and dismemberment is delivered to Shiroshi's door, and he and Seiji set out to investigate. They find themselves venturing to a remote island dominated by a towering Baroque mansion, where all manner of surprises await them - including a beautiful living doll and a young boy who calls himself Shiroshi's brother!