Evershade Valley is the main setting for Luigi's Mansion 2.
Evershade Valley with the Dark Moon intact, before the events of the game
It consists of five mansions that Luigi must explore, each having a separate them from the other.
Story
The Dark Moon hangs above Evershade Valley in the center, causing all the ghosts to be peaceful in nature. However, when King Boo destroys the moon, not only do all the pieces scatter to the five mansion, but the Valley becomes engulfed in a violet fog. This also causes the ghosts to go hostile, and cause mischief all over the mansion.
The valley after the break of the Dark Moon
Evershade Valley has five parts to it: Gloomy Manor and E .Gadd's Bunker are in a grass plain, with trees dotted around the place. Gloomy Manor has a typical mansion feel, being made up of normal rooms. Haunted Towers is placed in a very dense forest. It was once a greenhouse for plants before it was abandoned, and ever since it has been overgrowing with plant life. Because of the plants, most of the man-made portions have gone into despair, and everything is falling apart. The entire power supply of the mansion is powered by two waterwheels. There is also a giant tree growing between the towers. The Old Clockworks appears by a canyon, and is built over an ancient archeological digging site. The land is barren and sandy there, and there is little plant life. Everything with in the Clockworks is broken down and buried in sand, and the digging site goes very deep underground, to the point where E. Gadd can barely reach Luigi via DS. The Secret Mine is places atop a snowy mountain. Before it was abandoned, it was used by geologists and even smugglers would hide out there. Most of the mansion is underground, with everything covered in an icy coating. The parts that are above-ground have mostly been destroyed and are nearly non-functional. And finally Treacherous Mansion is located on a single piece of land, surrounded by a large chasm. It was once a museum with exhibits that showcased many artifacts from Evershade Valley and beyond. It is now overrun by ghosts, and has the highest paranormal signal of anywhere in the Valley.
The sole reason E. Gadd came to the Valley was to use the rare chance to study ghosts in a peaceful environment, and he called Luigi to the Valley to help him put the Dark Moon back into the sky.