THE GAMESTERS

The manga's story begins in a 60-year-old high-class apartment building, where a millionaire named Grant lives. One day, the apartment's doorman Howard reaches out to Grant, telling him that he wants to save Erica from the building's sixth floor from a cult. At first Grant will have none of it, until Howard reveals to Grant that he has knowledge of one of Grant's secrets.

(Source: Anime News Network)