Riddle me this, riddle me that. What do you have to buy, to play the Harley Quinn Expansion Pack? The Batman: Arkham City Game of the Year Edition!
Truthfully, the Harley Quinn's Revenge expansion will more than likely release as DLC, but as of this writing, it's only been announced for the Batman: Arkham City Game of the Year Edition thus far. [Update] It's releasing on May 29th on PSN and XBLA individually.
Warner let the
cat clown out of the bag hilariously tiny car early this morning with a trailer. The trailer confirms that the GOTY edition will include the Catwoman DLC, Nightwing DLC, Robin DLC, Batman Skins and Challenge Maps, as well as a movie download of Batman Year One; and of course, Harley Quinn's Revenge. The trailer also calls Harley's expansion the "final chapter", so don't expect more DLC after this. See the full trailer below.
Just to be clear, in the Harley Quinn's Revenge Expansion, you don't actually get to play as Harley. It's an all-new storyline complete with new areas and thugs, using Batman and Robin.