You have to admire Warner Bros. In this era of DLC backlash, especially when it seems designed to trick the consumer into spending more money beyond the retail price of the game, the way they've brazenly marketed the Harley Quinn Revenge Pack as "the final chapter of Batman: Arkham City" is a sight to behold.
They are selling the true ending as premium DLC, perhaps the most offensive use of premium DLC, and they're proud enough of that fact to, essentially, brag about it. Amazing.
Whatever your particular thoughts are on the issue, the fact remains that Batman: Arkham City's final chapter will send players back into Arkham City to stop whatever nefarious plan the Joker's number one fan is working on. During the two added hours of gameplay, players will switch between Batman and Robin and do whatever is necessary to stop Harley Quinn.
Furthermore, if you haven't yet picked up Batman: Arkham City, you should definitely check out the Batman: Arkham City Game of the Year Edition that is out on May 29th. It includes all previously released DLC (Catwoman Pack, Nightwing Bundle Pack, Robin Bundle Pack, Challenge Map Pack, and the Arkham City Skins Pack along withHarley Quinn's Revenge Pack).
If you already own the original, then on May 29th you can simply download Harley Quinn's Revenge Pack for $9.99/800 Microsoft Points.