Asked in Pokemon Diamond Pearl and Platinum
How do you get Spiritomb in Pokemon Diamond version?
If you need to complete your Sinnoh Pokedex, then all you have to do is battle Cynthia (she has Spiritomb).
If you need Spiritomb as one of your Pokemon, you need to go to Route 208 (west of Hearthome City), and talk to the guy who looks like a Black Belt Trainer (he is in the bottom right-hand corner, south of the Berry Master's house). The Black Belt will give you an Odd Keystone. There is another way to obtain an Odd Keystone, but I've forgotten it.
Then, go to Route 209 (east of Hearthome City). Keep going west until you see a pile of rocks. This is the Hallowed Tower. Go up to it, and press A. The game will ask you if you want to insert the Odd Keystone. Choose yes, then go underground and talk to 32 people (not including hikers, sorry guys!). The game can be fooled though, if you go underground, talk to one person, go back up and save your game. Do this 32 times, and the game will think you're talking to 32 different people!
Anyway, when you've talked to 32 people, go back to the Hallowed Tower, and Spiritomb will appear. I recommend taking lots of Ultra Balls and some Timerballs if you want to catch Spiritomb, as it is fairly difficult to catch.