Two Worlds Downloadable Content
Description: Four action packed multiplayer co-op maps enable you to explore new locations. Over 45 news quests, harder enemies and hidden bonuses wait to challenge you. This pack includes a new town that features a Merchant and Gladiators district, with advanced trainers to learn new skills and try them against opponents. Play with friends, you will need them.
I recall mixed reviews about this game when it came out; I've never actually played it myself. The pemise behind it sounds fun in theory, but seems like it could never really be implemented well.