You must have the HM Cut, as well as the Forest Badge, to find this easter egg. Use Cut on the trees near the exit to Eterna Forest to discover a path to a mysterious mansion. There are two rooms that hold spooks: One is the dining room downstairs and the other is upstairs on the far right, but you can only see it from the adjacent room to the left.

If you have a GBA game in slot 2 of the DS or the National Dex, you can get a Haunter and Gengar in the room with the "eyed portrait" that seems to look at you if you back away from it, or a Rotom in the 2nd floor TV (respectively).

Submitted by Manakoro Hashman