Batman: Arkham City
One thing about Batman villains is that they always keep coming back for more. After the events of Arkham City, Harley Quinn takes on the caped crusader yet again in this DLC pack.
During this story-based mission (which Warner Bros. claims lasts over two hours), players alternate control between Batman and Robin. Harley Quinn's Revenge is also the last DLC installment for the game, though players can get the complete package on May 29 with the release of Arkham City's horribly boxed Game of the Year edition. If you just want the Harley Quinn content, you can download it for $10 on May 29.
Get an idea what you'll be buying with these screens and a trailer: