The Dark Moon is an actual moon that is seen in Luigi's Mansion: Dark Moon. The Dark Moon resides over Evershade Valley and the Mansions that reside in it.
The Moon gives off an effect that calms the ghosts in Evershade Valley and makes them friendly. In the PAL version the Dark Moon is used as a satellite by Professor E. Gadd to calm the ghosts. King Boo destroys the Dark Moon, shattering it into six pieces and being scattered over the five mansions; Gloomy Manor, Haunted Towers, Old Clockworks, Secret Mine, and Treacherous Mansion. The sixth piece lands between Professor Elvin Gadd's Bunker and Gloomy Manor; the professor collected it before calling for Luigi, who must recover the other five pieces and make the ghosts friendly again.