The faction is led by Tyber Zann, a criminal overlord who has managed to get on the bad side of Jabba the Hutt. In fact, his ultimate goal is to take Jabba, as well as the rest of the galaxy, down. Unlike the existing campaigns in Empire at War, the expansion's campaign will be heavily story driven to introduce the new faction and all the new characters. The campaign begins with Zann in prison (the infamous spice mines of Kessel), and the first mission involves breaking out of jail, which is a good introduction to how the Underworld differs from the other two factions.
That's from the July 12th Gamespot preview. I thought that some of the questions/comments posted above would benefit from this information. The rest of the preview article can be found here:
http://www.gamespot.com/pc/strategy/starwarsempireatwarforces/news.html?sid=6153906&mode=previews)
Like in any good prison tale, Tyber learns of a major score from one of his cellmates, one that would make him rich and powerful if he has the guts and brains to pursue it. It won't be easy, however; to achieve his goal of controlling the galaxy's criminal underworld, Tyber will have to face off against such personalities as Jabba the Hutt, Prince Xizor of the Black Sun, Rebel hero Garm Bel Iblis, and Imperial mastermind Grand Admiral Thrawn.
That's from the most recent designer diary, also on gamespot. It seems that Black Sun will be an adversary of the Zann Consortium. The diary also says that the Eclipse SD will not be complete, but will still play an important part in the story. Kinda sounds like the death star 2 in episode 6.