The Sith Empire is one of two factions in The Old Republic. The Sith use brute force and sheer will to conquer and rely on the Dark side of the Force for their power.
The Sith were at first a little known race on the planet Korriban, until Dark Jedi discovered the planet and interbred with them. This formed a new, powerful civilization which expanded rapidly and eventually pushed into Republic Space, starting the Great Hyperspace war.
Dark Lord Naga Sadow led his Sith forces to destroy the Republic, but the Jedi repelled the Sith invaders and eventually destroyed their civilization on Korriban. However, Naga Sadow escaped into unknown space with his most trusted Sith. These survivors began rebuilding their civilization, unbeknown to the Jedi.
When the Sith were strong enough, they launched a massive assault onto the Republic. They began by infiltrating smaller star systems and making deals with crime lords. When the time was right, an all out offensive was launched and the Jedi were caught off guard. The Sith seized control of several star systems right off the bat and many more fell to their control in time.
The Republic leaders were surprised when the Emperor engaged in peace talks, but this was yet another trap as several Sith Lords and a host of Sith sacked Coruscant. With the Jedi Temple destroyed and the city in ruins, Republic leaders had no choice but to surrender several key star systems.
After the treaty was signed, the Sith Emperor went off to pursue his own goals mysteriously, leaving his Sith Council in charge of his Empire. The strongest will rise to the top to control the Empire and crush everything in their path.
