The Orion Syndicate was, at the inception of TOS TrekMUSE in 1992, one of the four original empires on the game as conceived by MacGyver and the original Directors. Despite beginning as one of the big four with the Federation, Klingons, and Romulans, the Orion Syndicate was doomed to dissolution before resurfacing, in substantially reduced form, as part of the Federation. The only race in the Orion Syndicate was, as the name strongly suggests, the mysterious and brutal Orions.
It will take somebody a fair bit older than the original author of this article (as incredibly implausible as that thought may be) to tell the story of the original Orions. However, it cannot have been a very successful lifetime, as the arrival of the Tholian Hegemony on the scene in 1993, from what very little information is available, seemed to sound the death knell for the Orions as an independent power.
(Comments from requested older author follow.) The actions, makeup and behavior of the original Orions remains lost to the early years of TOS lore. What is known is this: the Orions at one time had their homeworld in the Rigel system, and were centered mainly on Rigel VII. When they were seen as no longer viable as a separate entity, they folded briefly into the Klingon Empire before leaving the Klingons for the Federation. This sparked a war between the Federation and the Klingons in mid-1995, mainly over control of the Rigel system. The Federation claimed victory in this war (which was fought mainly during a period in which the Space system was down for upgrades) and the Orions survived for a brief period as part of the Federation. Declining membership and activity plagued them, however, and their double-dealing ways soured the Federation Council on continued support of the Orions, and they were cut loose to defend themselves. At this time, the organization was officially disbanded and ceased to exist on TOS until 1999. The Klingons took this moment to declare genocide against the Orions, laying waste to Rigel VII and exterminating surviving Orion members, while confiscating their ships. This was mainly a device to phase the org formally out of existence. It should be noted that for many years afterward, members of the Klingon Empire had standing orders to kill any Orion on sight.
During the intermediary years between the Orions demise as a great power and their return to marginal galactic prominence, they were occaisonally used as a minor plot device by admin looking to give their players something to do. The image as Orions as pirates, raiders, and uncouth mercenaries was a useful one in a galaxy where almost all of the elements were strictly organized.
In the latter years of TOS TrekMUSE thus far, the Orions resurfaced (largely through the efforts of Jacien in or around 1999) as a small raiding group, based out of the starbase Resonance. The assumption was that these Orions were the survivors of the Klingon genocide, and were now homeless and desolate. They appealed to the Federation for financial support and other considerations, claiming the Klingon genocide was the direct result of the Federation's withdrawal of support for the Orions. Gradually, they rebuilt their power base.
The Orions and their deadly cruiser the OIV KageKuro were alternatively allies to the Federation and one of Starfleet's most bitter enemies, frequently jumping Starfleet vessels while they were in space and, in one memorable battle, destroying the storied USS Excalibur under the command of Suvin Grey. For the most part, however, the Orions have been nothing but pesky raiders and occaisonal trading partners, and the Federation has often reacted with displeasure when one of the other empires (usually the Klingons, who have had a bone to pick with the Orions since time immemorial) incurs on Orion assets.
The Orions have undergone a great deal of turnover recently. Their little empire, even by their modest standards in the wake of coming apart in TOS's early days, has deteriorated badly, and their tinpot navy is no longer up to Starfleet's standard. Several casualties have put the leadership of the Orion Syndicate in doubt, and the plucky little slave drivers will need to pull up their bootstraps in order to return to prominence.