What is the name of the first DLC expansion for Batman: Arkham City?

The first story-based DLC expansion for Batman: Arkham City was Harley Quinn's Revenge. It follows the events of the main game, after Arkham City has been evacuated and Batman disappears while investigating Harley Quinn's activities.

Released in 2012, the expansion allows players to control both Batman and Robin. Robin's role is especially important because the story centers on his search for Batman and his attempt to stop Harley's revenge campaign. The campaign adds new areas, enemies, gadgets, and story missions rather than merely supplying challenge maps.

A common mix-up is Catwoman's DLC. Catwoman was downloadable content tied to the game's launch and included a parallel campaign, but Harley Quinn's Revenge is generally identified as the first major post-release story expansion. Nightwing and Robin were also added as playable characters through separate DLC packs, while The Batmobile was not a playable feature in Arkham City.