You need an Odd Keystone. You can get one from a man on Route 208.
Then put the Odd Keystone into the broken tower on Route 209 to make it the Hallowed Tower.
Greet 32 people in the Underground. It can be the same person but one of you needs to leave and re-enter to get it to count.
Go back the Hallowed Tower and press A. Spiritomb appears at level 25.