From the Super Mario Wiki, the Mario encyclopedia
The Haunted Catacombs are a set of areas found in Treacherous Mansion in Luigi's Mansion: Dark Moon. Located in the basement and accessible from the Ceremonial Chamber, Underground Lab and Dungeon Cells, they're both maze like areas where most paths simply bring Luigi back to his starting location.
In the first set of Haunted Catacombs, there's a throne that leads back to the Ceremonial Chamber, a coffin that opens into a Bonus Challenge and some suits of armor that try to strike Luigi as he runs by. This area leads to the Underground Lab via a door to the right.
The second set of Haunted Catacombs has a different maze layout, and has multiple suits of armor that appear to be pointing in different directions. Following these directions leads Luigi to a secret area, where one can find a fire painting and a spider web ball. Other paths lead him back to a lower level of the Cliffside, whereas the main route eventually leads him to the Dungeon Cells.
In both areas, the Dual Scream's map is simply blank.
During Front-Door Key, Luigi had to navigate both sets of Haunted Catacombs to track down the Polterpup that had escaped with the key. He did this by following the dog's paw prints with the Dark-Light.
During Terrifying Invasion, the Haunted Catacombs can be entered again, along with the entire underground section of the mansion. They have no purpose however, and for unknown reasons many suits of armor and all gargoyle heads originally found here are gone.