From the Super Mario Wiki
Evershade Valley is the main setting of Luigi's Mansion: Dark Moon. It is separate from the Mushroom Kingdom and is known both for its large number of ghosts and as the location of the Dark Moon, a celestial body that has a pacifying effect on ghosts. The valley itself is separated into different areas, with each containing a mansion inhabited by ghosts.
Sometime after the events of Luigi's Mansion, Professor E. Gadd established a laboratory in the valley to continue his studies on ghosts. While there, E. Gadd obtains the help of the ghosts from the various mansions in the valley for his studies thanks to the Dark Moon's pacifying effects. One night, however, King Boo materializes in the shadowed sky and destroys the Dark Moon and scatters its shards across the valley. With the Dark Moon gone, the ghosts become hostile, which forces E. Gadd to recruit Luigi in order to capture the ghosts and restore peace to the valley.
Names in other languages