First go to Solaceon (spelling??). Go south past the tower and you'll see this rocky circular thing (it's called Hallowed Tower) talk to it and you put the Odd Keystone inside. Now you have to talk to 32 people underground. This does not include hikers. If you have a friend, you can use him/her. If you talk to your friend underground, you both leave and go back down, it will count as another person when you talk to them. After that talk to the Hallowed Tower and Spiritomb will attack.