Dark Age Exhibit
From the Super Mario Wiki
The Dark Age Exhibit (Dark Ages Exhibit in British English versions) is a room in Treacherous Mansion in Luigi's Mansion: Dark Moon. Located at the bottom left of the map, it is accessible through either a north door via Grand West or the east door from the West Gallery. The room is a museum exhibit based on the Middle Ages with various suits of armor and medieval objects littered around, and is a reference to Gloomy Manor in design.
The northeast corner of the room has an E-Gate leading back to the Front Entrance, the middle of the room has a bookcase with a gem behind it, and there is a treasure chest to the right whose contents vary by mission.
During Double Trouble, the Dark Age Exhibit is one of the areas Luigi has to explore and clear of ghosts. Examining the iron maiden to the northeast causes two Strong Hiders to appear, and opening the treasure chest gives Luigi a book which can be placed in the back bookcase to lower it and reveal the E-Gate. There is also a Strong Greenie here wearing a bucket as a helmet; this bucket can be used to solve puzzles in the Jungle Exhibit and the Ancient Exhibit.
From A Train to Catch onwards, the bookshelf to the northwest has been already lowered and the E-Gate is accessible by default. Taking a Bulb here from the Study lets Luigi reach the upper part of the room and find a Gold Fruit; using this reveals coins in the shape of a Boo and reveals the Boo for this mission once the coins are collected.
In Terrifying Invasion, the Dark Age Exhibit may be one of the rooms infested with ghosts.