Although Nightwing will be playable in challenge maps only, much like Tim Drake's Robin, he can be used in any of the maps already on the disc or downloaded previously as well. you also get a pretty cool little extra in the form of an animates series style skin.
Warner Bros. Interactive Entertainment and DC Entertainment today announced the Nightwing Bundle Pack for Batman: Arkham City will be available November 1st. Nightwing comes complete with his own unique gadgets and special moves, and is playable in all challenge maps in the game, as well as two additional challenge maps that are included with the pack: Wayne Manor and Main Hall. The pack also contains a bonus Animated Series Nightwing character skin.