Very little is known of the Sith before they made contact with the Republic to initiate the Great Hyperspace War. By luck or by the Force, two young hyperspace navigators stumpled upon the Sith Empire in the midst of a power struggle. The reigning Sith Lord had just become one with the Force leaving a power vacuum and two Sith vying for the throne: Naga Sadow and Ludo Kressh. Sadow used the arrival of the young navigators to his advantage, dividing the ranks of the Sith and preaching the dangers that were soon to follow.
Disguising his invasion of the Republic as a preemptive strike against a soon-to-be attacking enemy, Sadow had the Sith Empire make ready for war. However, due to the valiancy of the Jedi Knights, soverign protectors of the Republic, and in-fighting between the Sith themselves, the Sith forces were pushed back, forcing Sadow to retreat and remain on the fourth moon of Yavin until his death.
A millenium later, a young Jedi by the name of Exar Kun, powerful in the ways of the Force, set out on a quest to learn the secrets of the Sith, the forbidden knowledge withheld from him by his Master. Kun believed that he could resist the temptation of the Dark Side while learning its secrets. However, the Dark Side's pull is strong indeed, and Exar Kun became one of the worst threats the galaxy had ever seen. His campaign to revive the Sith Empire and declare himself the new Sith Lord once again thrust the galaxy into conflict; conflict that destroyed many worlds, killed many Jedi, pitted brother against brother, student against master. In the end Kun was defeated and the Jedi were left to rebuild the galaxy in the wake of Kun's wrath.
Unfortunately, in the result of the previous war, the galaxy was unstable at best, and decades later, the Mandalorians took their shot at domination. Once again the Jedi were called to fight in these Mandalorian wars and once again, out of fire, a resurgence of the Sith was born. Two of the Jedi Order's best, Malak and Revan, who led the Jedi forces against the Mandalorians, were consumed by the Dark Side themselves. Declaring themselves the new Sith Lords, Malak and Revan once again plunged the galaxy into war. While ultimately defeated, an aftershock in the form of Darth Nihilus and Darth Sion sent the Jedi into hiding as the Sith implemented a sytem of extermination and genocide against the Jedi. The Sith's defeat at this junction would give the Jedi almost two thousand years to rebuild their order from such devastation.