K no Souretsu

A resident of Room 301 in Building 67, K, had died and no one had seen or was able to find his corpse. Everyone, however, was still certain that he was dead and held a funeral for him. That was when a stranger named Mikaya suddenly showed up and announced that he was going to live in 301. Determined to solve the mystery of this far from ordinary death, Mikaya began to question the residents and as everything began to unravel, he realized that something was very, very wrong.

(Source: Anime News Network)

Note: Includes one prologue and three extra chapters.