Treacherous Mansion

Treacherous Mansion is the final and longest mansion that Luigi visits in Luigi's Mansion: Dark Moon. It has a museum theme. It is also the location of the final Dark Moon piece. Many rooms on the first floor of Treacherous Mansion are based off of the previous mansions in the game. Almost every ghost that Luigi encounters appears here. King Boo comes to this mansion to open a paranormal portal to unleash almost every ghost that Luigi has ever encountered, such as Greenies, Slammers, a Gobber, Sneakers, Mummies, Hiders, and Creepers, and most of these ghosts are either shielded, strong, or equipped with a weapon of some form. This multitude of ghosts will bring about the destruction of Evershade Valley. There are also gargoyles inside and outside of the mansion.

Treacherous Mansion is the only mansion in the game that requires Luigi to unlock just one door; the rest of the doors are already unlocked. It is also the only mansion besides Gloomy Manor and Old Clockworks that has multiple music tracks; the track changes whenever Luigi looks through a window or crack in the wall. It is also the only mansion to have two missions primarily dealing with ghost-capturing: Paranormal Chaos and Terrifying Invasion. It is also the only mansion to have multiple trapdoors. Finally, it is the only mansion whose respective Dark Moon piece and possessor ghost are not found in the mansion's vicinity.

The bottom four exhibits in the mansion are based on the four previous mansions in the game in some way. The Dark Ages Exhibit represents Gloomy Manor due to the knights found in the manor (despite not having the same music tracks), the Jungle Exhibit represents Haunted Towers due to both having botanical designs, the Ancient Exhibit represents Old Clockworks due to the latter having an Ancient Egyptian-like basement, and the Ice Age Exhibit represents Secret Mine for their icy themes.

History
Treacherous Mansion is an abandoned Museum positioned right in the middle of a giant ravine, which becomes cut off from land access after Luigi reaches the front steps. According to E. Gadd, the mansion contains artifacts and items from Evershade Valley and beyond making it the most "precious" of the mansions. The Mansion is slightly changed to a more spooky feel after King Boo arrives. It is called the Treacherous Mansion as it could collapse into the ravine below. It however does not at any point.

In-game description
''This massive mansion is filled with exotic exhibits created by its late globe-trotting owner. Oh, and it also contains the strongest paranormal signals ever recorded!''

''This massive mansion is filled with exotic exhibits fashioned by its globetrotting late owner. Oh, and it also has the strongest paranormal signals ever recorded!'' (PAL version)

Treasures

 * 13 Diamonds
 * Keys

Bosses

 * Mini-bosses: Strong Poltergeist, Big Boo
 * Bosses: Tough Possessor (possessing suits of armor)

First Floor
(Ground Floor in PAL version)
 * Veranda
 * Front Entrance
 * Inner Courtyard

First Floor West Side
(Ground Floor West Side in PAL version)
 * West Gallery
 * Dark Age Exhibit (Dark Ages Exhibit in PAL Version)
 * Grand West
 * Kitchen
 * Ice Age Exhibit

First Floor East Side
(Ground Floor East Side in PAL version)
 * East Gallery
 * Jungle Exhibit
 * Grand East
 * Restrooms (Toilets in PAL Version)
 * Ancient Exhibit

Second Floor
(First Floor in PAL version)
 * Inner Courtyard
 * Terrace

Second Floor West Side
(First Floor West Side in PAL version)
 * West Corridor
 * West Balcony
 * Dark Age Exhibit
 * Space Exhibit
 * Archives
 * Aviation Exhibit
 * Ice Age Exhibit

Second Floor East Side
(First Floor East Side in PAL version)
 * East Corridor
 * East Balcony
 * Jungle Exhibit
 * Nautical Exhibit
 * Study
 * Train Exhibit
 * Ancient Exhibit

Third Floor
(Second floor in PAL version)
 * Gargoyle Roof

Basement 1

 * Veranda
 * Bottom of Well (Bottom of the Well in some regions)
 * Guard Tunnel (Guarded Tunnel in PAL Version)
 * Ceremonial Chamber
 * Cliffside
 * Jungle Exhibit
 * Ancient Exhibit

Basement 2

 * Dungeon Cells
 * Haunted Catacombs
 * Underground Lab
 * Cliffside

Basement 3

 * Dungeon Cells

Trivia

 * This is the only mansion where Luigi has to rescue two Toads in one mission.
 * While Luigi is under the mansion in the Dungeon Cells, Professor E. Gadd will call him, but with no interference with the DS.
 * The Treacherous Mansion is likely based on William Hope Hodgson's 1908 novel House On The Borderland, in which the denizens of the titular house are assaulted by creatures from another dimension. Similarly, the house in the novel gets a large portion of the ground below it spirited away, leaving the house stranded on a ledge directly above a large, water-filled ravine.