There are many hidden objects in the Sinnoh region of Pokémon Brilliant Diamond and Shining Pearl, and the Odd Keystone is one of them. Just out of sight, this item is frustratingly easy to find if you know where to go. This item will eventually lead you to get Spiritomb.
The Odd Keystone can be found by following Route 208, just before entering Hearthome City. It is also east of Mount Coronet. Once you reach a grassy stretch to the left of the Route 208 sign, you will see a martial artist at the bottom right of the screen. It is hidden under some pesky trees. Go talk to him and he'll give you the strange keystone.
So it tells you to go to Route 209 and go to the Grand Underground in that area. The Odd Keystone is used to prevent the collapse of a stone tower in the area. But first, you need to find the Hallow Tower just south of Solaceon Town on Route 209. Once you touch it, it will ask you to use the Odd Keystone.
The martial artist tells you to listen to the stone pillar once you are over there. Route 209 is east of Hearthome City. If you have forgotten how to go underground, use the Explorer Kit under “Key Items Pocket” in your bag. When you reach the subway, he talks to 32 different NPCs or 32 online players.
Once this task is complete, return to the Hallow Tower. He will then trigger a battle with Spiritomb. Be sure to save before this encounter so that you can give yourself a better chance of catching the Mythical Pokémon. Also stock up on Pokéballs.