The Underworld is not a government. It is not an organization, syndicate, or any structured group. There is no one leader of the Underworld, there is no ruling council, and there is no one goal or vision, save survival.
The Underworld is a loose affiliation of criminals, scoundrels, scum, and all those who would dwell within the darker underbelly of the galaxy.
As it’s name implies The Underworld is all encompassing, neither rejecting nor accepting anyone or anything. In simplest terms The Underworld is a compact under taken by dozens of gangs, syndicates, organizations, and even individual criminals. Though there is no real constitution or even binding the agreement The Underworld works on the simple thought of cooperation out of necessity.
With the galaxy filled to the brim with factions of Jedi, Sith, and Empire’s the little guys began to suspect they were simply outmatched. Those entrepreneurs who sought a simple living in the trading of spice found themselves arrested more often, those selling their services as assassins found themselves stopped by the more organized Jedi, and even the common thug found himself thrown to the ground for simply plying his trade. It became more and more obvious that the dark of the galaxy was being snuffed out, and thus The Underworld was formed.
There is no real organization or structure to the Underworld, instead it is a compact. The idea is simple. When faced with a larger threat, those within The Underworld band together to protect their interests as a whole.
The idea is simple, and surprisingly effective. A man within the Underworld might be on the run from a band of Alliance Soldiers whom he stole from, he comes upon an establishment that bears the mark of the Underworld, he offers a portion of his gain for protection and is quickly given it. The Establishment hides the man and garners a portion of his ill-gotten gains. This same concept poses to bigger problems as well. A world largely controlled by the galaxies biggest spice dealer is invaded by a galactic peacekeeping force. Pirates, swoop gangs, and bands of thugs band together to protect the Spider Dealers stock. After the invasion finishes, each group that came to the defense of the world receives a portion of the stock of profit.
This idea is thus, simple. The Underworld comes together in order to survive the larger threats of the galaxy.
Of course, The Underworld is still entirely built on Criminals, and as we all know Criminals are not often the most trustworthy. Subterfuge, plots for power, and outright betrayal are not uncommon within The Underworld. For instance in the explanation above the largest of the Pirate groups who came to the rescue of the Spice Dealer may simply decide to...reappropriate the Spice Dealers wares for themselves.
Such is the risk of The Underworld. Everyone plays together, but they don’t always play nice.