The Sith Empire, later the Old Sith Empire, was a rich and powerful empire established by Dark Jedi who were banished from the Galactic Republic after their defeat at the Battle of Corbos during the Hundred-Year Darkness.
The Jedi exiles fled to the remote world of Korriban, where, after conquering it, they interbred with the native Sith species through the art of Sith alchemy, beginning a long period of building and expansion. The Sith Empire developed in complete isolation for two thousand years before it eventually rediscovered the Republic.
Under the leadership of the Dark Lord Naga Sadow, the Empire invaded Republic space during the Great Hyperspace War, but it was defeated. The Sith Empire was believed to have been destroyed by Republic forces during the second battle of Korriban.
However, the Sith Lord Vitiate survived the destruction and led many of the remaining Sith in rebuilding the Empire in the Unknown Regions. Though the Sith Empire was defeated, it served as an example to be followed by numerous Empires of later eras, with a legacy that extended across millennia.