Last year we got a brief look at Crawl which has the most ingenious multiplayer system I've seen in a long time. One player will take on a courageous hero, hacking and slashing his way through a dungeon whilst the other players will take control of evil spirits trying to stop him. They can take charge of traps or monsters to slay the hero and if they do, they'll become the hero.
From the trailer above, it looks like there might be some frantic and frenzied action to get the final blow in so that you can become the hero. I thought Crawl might be more co-op focused but I think having the evil players competing against each other as well as the hero player might make this a really unique experience.