Location of Haunted Mansion
The official name of the haunted mansion is the Old Chateau. To access the Old Chateau, travel to Eterna City, located west of Celestic Town. Exit Eterna City using the western exit, and follow the road west on Route 205 to reach the entrance to the Eterna Forest. Immediately after entering the forest, use the Pokemon move "Cut" to cut down the small bushes to your north. After cutting down the bushes, follow the road north until you reach a large house, which is the Old Chateau. Walk through the double doors to enter the mansion.
Pokemon Found in the Haunted Mansion
Inside of the mansion is only one Pokemon, which is Gastly. For "Pokemon Diamond" users who are not very far in the game, this is the first Ghost-type Pokemon they have the opportunity to catch. Gastly's levels vary, ranging from 12 to 16. However, if you are playing on a Nintendo DS or DS Lite and insert any Generation III game into the Game Boy Advance slot, such as "Pokemon Ruby," you can find Haunter in the mansion instead. Haunter, the evolved form of Gastly, only appears as a Level 16 Pokemon. As a Ghost-type Pokemon, both Gastly and Haunter work well against Psychic- and Ghost-type Pokemon.
Items Found in the Haunted Mansion
Inside of the Old Chateau are three items: two unique items and one common item. The common item is an Antidote, able to heal your Pokemon of the poison status effect. The Dread Plate, located on the second floor in the last room on the left side of the Old Chateau, is an item that any Pokemon can hold to increase the power of its Dark-type moves. The last item is TM90, which teaches a Pokemon the move "Substitute." TM90 is able to be found on the first floor in the last room on the right side of the Old Chateau.
Rotom
Once you have defeated the Elite Four and received the National Dex, it is possible to catch Rotom in the Old Chateau. Rotom, a Ghost- and Electric-type Pokemon, can change forms and alter its type. Enter the Old Chateau at anytime throughout the night, and go through the northern door at the top of the stairs. Enter the second door on the left; inside of the room is a television that is turned on. Press "A" next to the television to engage in battle with Rotom, who is Level 15. If you defeat Rotom instead of capturing it, you must come back the next night if you wish to try again.