Xbox has announced the first update for Halo: Campaign Evolved since August, one which will add Firefight to the remake's overhaul of the original game's story.
Firefight is Halo's cooperative PvE mode which debuted in Halo 3: ODST. It's a round-based defence affair where players must survive as long as possible against increasingly dangerous waves of enemies. Following ODST, the mode appeared in every subsequent Halo game with the exception of Halo 4, which featured a mode called Spartan Ops instead.
