The Haunted Mansion is a location within the Haunted Forest, in the Rijon region in Pokemon Prism. The player must clear the Haunted Mansion before they are allowed to challenge the Torenia Gym in Naljo.
The front door to the Mansion is open, but the door into the mansion proper requires a Mansion Key. This can be obtained by touching the colored graves in the Haunted Forest in the correct order, as given by the NPC in the mansion's first room.
Once the interior door is unlocked, the inside of the mansion can be explored. To progress, the player has to enter the locked bedroom upstairs, which requires finding two Bedroom Keys. These are located in chests on the first and second floors (see Item table below). The bedroom itself is on the second floor, behind the locked door in the middle.
Inside the bedroom is a Gengar, which, when engaged, sends the player to a dream dimension. There are no battles or pokémon here, but there are various NPCs to speak with. To progress to the second map, the player needs to speak to the NPC who is a duplicate of themselves. After exploring both maps and speaking with its occupants, the player character will awaken in bed, with the Gengar having fed on their dream.