Marriland shows you where to find the Pokemon Rotom inside of the Old Chateau.
This was recorded on my Japanese Diamond version because I haven't gotten that far on my English version yet and I needed to write about how to find Rotom.
I can't believe that I was so stupid to forget that Rotom was completely immune to all of Quagsire's attacks, haha! I was going to reset but was surprised that the first Ultra Ball caught it, so I decided to keep the video. :p
You can check out Rotom's Pokedex information, as well as information on every single Pokemon in the game, at:
It has a full Pokedex and complete Walkthrough so you'll know everything there is to know!