For the first time in the series, Dawn of War II's next expansion will let you play as someone other than the forces of the Imperium. Here's video of evil Space Marines as proof.
The forces of chaos are one of the new playable factions in Dawn of War II, another being the Orks, along with the Eldar and the Tyranids. While previous games and expansions have let you play as different races in multiplayer, this is the first time we've been able to take control of them in a singleplayer campaign.
The name of this expansion is Retribution, and it'll be out in March.