What do you do with the "old keystone" in Diamond?

Hi there. Once you get an Odd Keystone, Fly to Solaceon Town and walk south until you reach that tower that looks like it has collapsed. Use the Odd Keystone on it and it will turn into the Hollow Tower.

Then, go into the Underground and meet 32 people. After you do this, go back to the Hollow Tower and go Underground right next to it and you should find a Spiritomb....although I have not done it myself (my friend traded me one from his game that he bred, and I then bred it myself).

Hope this helps :)

Found in Route 209.

You use it to get the Pokemon "Spiritomb", go to serebii or marriland and they should have it.

Hope this Helps XD

Celebrate

its on route 209... once you seen 32 people in the underground you can catch it.