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Pokemon Diamond and Pearl: Rotom
Note: You must have the National Pokedex before you can catch Rotom.

Rotom can be found in the Old Chateau which is located in the northern-most part of Eterna Forest. You need to bring a Pokemon that knows Cut in your party to access the Chateau. Once you are inside, go up the stairs to the left or right and enter the middle doorway upstairs. In the next room there will be five doors. You must enter the door to the left of the middle door. There will be a flashing TV in this room. When you walk up to the TV and press A, Rotom will attack.

Rotom will be level 15 and is Electric and Ghost.