Autumn of the Republic
The Sith split off from the Jedi Order about 6,300 years ago. A group of Dark Jedi embraced the dark side of the Force, and after a brutal war, were exiled to parts unknown. The Sith Empire grew in secret until the Republic accidentally rediscovered them about 4,300 years ago, triggering a series of wars. Many times over the millennia, the Sith were seemingly defeated, only to return again and again.
What made the Sith so dangerous is that their embrace of the dark side was as strong as the Jedi’s devotion to the light side. The Sith developed alchemical technology to modify their weapons and mutate strange beasts, and their most powerful practitioners elevated their knowledge of the Force to sorcery — seemingly accomplishing the impossible. Further, many Jedi, even the most noble and stalwart, were corrupted into their ranks over the years, either by capture, attempting to infiltrate them from within (Ulic Qel-Droma’s fall centuries ago is a well-known cautionary tale), or trying to use Sith artifacts or powers, even for heroic purposes (Darth Revan, the Prodigal Knight, both saved and nearly destroyed the Republic 3,000 years ago).
Most recently, 300 years ago, the Sith were reborn as the Brotherhood of Darkness, a group of darksiders who invaded much of the known galaxy. The Jedi fought them to a standstill at the planet Ruusan, where the Sith used an ancient weapon of terrible power, which caused heavy Jedi casualties, at the cost of every Sith life on the planet. It is widely believed the Sith are well and truly dead now.
(GM’s Note: Of course, as people who have seen the Star Wars movies, we know the Sith aren’t really dead — they’re in hiding, and eventually, the Emperor and Darth Vader will destroy the Jedi Order and turn the Republic into the Empire. However, that’s not for another 700 years — at this point in history, the Jedi Order is strong, and the Sith are believed to be completely extinct. The Sith will not play a role in this campaign.)