From the Super Mario Wiki
“See that on the screen? That there is the Dark Moon. It's a mysterious celestial body that has a pacifying effect on all of the ghosts in Evershade Valley.”
The Dark Moon is a moon that is seen in Luigi's Mansion: Dark Moon. It resides in the sky over Evershade Valley and the five mansions in it. The Dark Moon exudes energy that calms the ghosts in Evershade Valley and causes them to be friendly. In the beginning of the game, King Boo summons power from the gem on his crown and zaps the Dark Moon, shattering it into six pieces that are scattered across Evershade Valley. One of the pieces lands between Professor Elvin Gadd's Bunker and Gloomy Manor; the professor collects it before contacting Luigi. Each of the five remaining pieces are given to a possessor ghost, by King Boo, in each of Evershade Valley's five mansions: Gloomy Manor, Haunted Towers, Old Clockworks, Secret Mine, and Treacherous Mansion. Luigi must recover all of these pieces and restore the Dark Moon in order to make the ghosts peaceful again. After a piece has been recovered, E. Gadd uses an invention to clean it in order to get rid of disruptive energy and restore its original properties.
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