The add-on is basically horde-mode where your gained experience and drops can be transferred over to the main story. It works in both single and multiplayer, and features a leaderboard, so you can see how efficiently you bash zombie skulls in comparison to your friends and family. It even includes a new weapon, called the Brain Wave Bomb.
Bloodbath Arena will be released for PS3, Xbox 360 and PC. It will cost ten dollars, or the equivalent in Microsoft’s silly money.