Rotom is a powerful Electric / Ghost Pokémon in Pokémon Brilliant Diamond and Shining Pearl. This Pokemon can take multiple forms and also learn many different moves. However, capturing Rotom can be tricky, as it requires checking multiple boxes. That said, let's look at what you need to do to get Rotom.
Before we get into how to get Rotom, we should note that two things need to be in place before the match. First, you must have beaten the Elite Four. Encounter with Rotom is part of the post-game content in Shining Diamond and Shining Pearl. Second, you must be at night.
To find Rotom in Pokemon Brilliant Diamond and Shining Pearl, we recommend that you go west of Eevopolis and on Route 205. West of the city and this path is the forest of Evisopolis.
Take a few steps west and you should see two trees and the Gym Leader Gardenia. Talk to Gardenia and she will tell you about the ghostly figures that roam around the Ancient Castle. From here, cut down the trees with the Cut HM and walk to the building at the end of the path.
This building is the Old Chateau, and this is where you will find Rotom. If you've played Diamond, Pearl, or Platinum, this might look somewhat familiar.
Now, to actually get to Rotom, you'll need to find the room that has the TV. To do this, go up the stairs and go through the door right in the middle of the second floor.
After you do this, you will notice that there are several rooms, including one right in front of the second floor entrance. Walk left and enter the room which is right next to the one opposite the second floor entrance.
In this room is the TV we talked about earlier. Approach and press A, and there should be a message asking if you want to hit the TV, since there appears to be a Pokemon. Select "Yes" and you will get the meeting with Rotom.
The Ghost / Electric Pokemon will be at Lv. 15, so be sure to be careful if you're trying to weaken it.