The earliest Sith lived on the red, dusty planet of Korriban, determined to grow strong despite the inhospitable climate. Their success culminated in a bold civilization, made stronger when dark Force-users arrived and interbred after being driven out of the early Jedi Order. This ancient Sith civilization eventually became a superpower and challenged the Republic during the Great Hyperspace war. Their defeat drove a handful of survivors into deep space where they regrouped and rebuilt, determined to return.
Over time, the new Sith Empire lost its way back to Korriban, until a visit a few centuries ago by two Jedi returning from the Mandalorian wars. Turned to the dark side, the two Jedi, Revan and Malak, attempted to reclaim the Sith's legacy in the known galaxy, but turned on each other and self-destructed.
The true Sith Empire, governed by a thousand-year-old Dark Emperor, wisely waited, and returned to the known galaxy only when completely prepared to defeat the Jedi and the Republic.
Retaking the holy planet of Korriban as a top priority, the Sith re-established the glorious Sith Academy and began training a new generation of Sith to inherit their dark legacy and seize their birthright as the true rulers of the galaxy.