THE DECAGON HOUSE MURDERS

The members of a particular university's Mystery Club travel to a ten-sided house on a remote island...which just happens to have been the scene of a gristly, and unsolved, mass murder. Armed with details of the murder and keys to the property, they set out to dig deeper...but can they trust the "facts" of the case? And moreover...can they trust each other? (Source: Kodansha USA) — Description from AniList community (anilist.co).