SPIRITOMB

Spiritomb might not be a Legendary Pokemon, but it's still a huge pain in the ass to obtain. To start up the sidequest to find Spiritomb, you'll first need an Odd Keystone. There are several of these scattered throughout Sinnoh. Two particularly easy-to-get Keystones are found in the small pond at the southern part of Twinleaf Town (use the Item Finder) and from the karate guy towards the end of Route 208. Once you have a Keystone, head towards Route 209 and find the odd-looking stone structure. (It's located right before the path swings north.) Walk up to it and press A to examine it, and then drop in the Keystone when prompted. Nothing will happen immediately, but by using the Keystone, you've started up the Spiritomb sidequest.

Now, for the next stage of this task, you'll need to find some friends in your area with copies of Pokemon Diamond/Pearl. Head to the Sinnoh Underground using the Explorer Kit, and have your buddies do the same. (Since the Underground is not available over long-distance Wi-Fi, you'll need to have your pals physically closeby). Find your friends in the maze and initiate conversation by walking up to them and pressing A, then choose to introduce yourself. Each player you talk to while underground will add onto the "People met" number of your Trainer Card. After you've talked to all of your friends, go back above ground, then use the Explorer Kit again. Once again, find the others underground and talk to them to add onto your "people met" total. You'll need to get this number up to at least 32, which can take a very long time with only one other person, so the more people you can get to go Underground with you, the better.

Once you've talked to enough people, pop on up to the surface again and find the structure you used the Keystone on earlier. If you've successfully made 32 contacts, you'll be attacked by a Spiritomb at about level 20. At this low level, it shouldn't be too tough to catch, but be careful not to botch it, especially after all this work.

If you have another keystone, you can restart the quest and get another Spiritomb by using it at the rock formation again, but it will require you to add another 32 people onto your "people met" number. At this point, you're much better off breeding if you want more of the little spooks.