I picked up a board game over the holidays: Risk Halo Wars. The Halo-themed version of the Parker Brothers classic turned out to be quite a bit of fun.
As you can see in this video, the game features three playable factions: The Flood, the covenant and the UNSC. Each faction has regular units, units worth three regulars and a single hero that adds a bonus point to your top roll.
The game can be played with classic World Conquest rules, or you can play with basic training or command center rules, both of which require you to earn a set number of in-game achievements to win.
Achievements include things like needing to control an entire region or a set number of enemy bases or taking over a number of colonies in one turn.
There's not a significant difference between how the factions play, though some get bonus cards and other get bonus units when you start.
If you already own Risk, this may not have enough different to make the purchase worthwhile unless you're a Halo fan. If you're looking to pick up Risk, though, I'd say get this one.