What do you do in haunted house next to eterna city in Pokemon diamond?

Firstly, make sure you have the National Pokedex (if you've seen all the Pokemon in your initial Pokedex, go back to Professor Rown and talk to him). Then wait until night-time. In the Old Chateau, there ought to be a room with a TV in it. Go up to the TV and press A; there is a Lv15 Rotom (Electric/Ghost type) inside. This apparently may take a few goes before Rotom appears, but it worked on the first try for me.

Rotom has the Ability Levitate and the attacks Thundershock, Confuse Ray, Uproar and Double Team, if this helps. It's incredibly easy to catch.

Other than that, I think there are a couple of items in the Old Chateau; nothing too major.

The items are a dusk stone and a charcoal. Use a quick ball by the way.^this is not the same guy. it's a legendary pokemon.