major console patch.
Bloodbath Arena is free to customers who pre-ordered the Special Edition game, and costs $10 or 800 MS Points for everyone else. It offers four new arenas for single-player or co-op play, and challenges participants to execute and outlive multiple waves of zombies. So, it's a survival mode in which you face off against a constantly replenished force of snarling enemies. You could call it "Mob Mode" or something like "Enough Enemies to Warrant a Collective Noun Mode."