Shadow Ball is the bane of any pokemon that uses its mind to attack opponents. One of the most effective ghost-type moves around, it's a great way to trounce no less than two of the Elite Four in Pokemon Black and White 2. Problem is, it's tricky to find unless you literally think outside the box.
Before you can get Shadow Ball, you need to gain access to Lentimas Town, found after you defeat fifth gym leaderSkyla. When you do she'll shepherd you across Unova in her personal plane, to Lentimas Town, just west of Reversal Mountain. Leave town to the east to find the short route leading up to the mountain. Going north takes you to the interior of the mountain... but going south and east, through a big patch of grass, takes you to the Strange House.
The Strange House is aptly named, as it's filled with ghostly apparitions and furniture that moves on its own. This is a fitting place for Shadow Ball, but the TM isn't actually in the house itself. Once you climb the small flight of stairs, go immediately left and around the fence. Follow the edge of the property to the north and try to go behind the Strange House. You'll be stopped by an unseen object. Pick it up to get Shadow Ball.