Dark Horse’s tales of the Jedi series dealt with the Jedi and the Sith 4000 years before the films. Now things go back a whole millennia earlier as a prequel to that series, published in 1998.
Basically, this reveals the origins of the Sith. Even further back, there’s a first ‘great schism’ between light and dark Jedi. These dark Jedi eventually are exiled to the other end of the galaxy, but eventually create their own civilization by conquering an alien race, the Sith. They become, in essence-the “Dark Lords of the Sith”-a legacy that would last for years to come, although in many different variations (Most notably, the movies’s “rule of two”).
However, they remain largely isolated from the rest of the galaxy. That is, until some explorers-charting new routes in hyperspace-discover them. An ambitious Sith Lord, Naga Sadow, uses this opportunity to depose his rival Sith Lords, and begins a war with the Republic. The two explorers-brother and sister are also pressed into the war-the Sister helping the Jedi and the Republic, the brother somewhat reluctant helping the Sith.
However, the Republic-helped by the Jedi Knight Odan-Urr’s warning of the Sith threat, as well as Empress Teta and her system-are able to fight back and repel Naga’s sith fleet. The brother also betrays Naga, causing him to flee.
However, Naga himself is able to escape to a small moon orbiting a gas giant-a moon that, in years to come, will be used by future Sith Lords, but will also be the home base of the Rebel Alliance-Yavin IV. He also constructs Sith Temples there-the same temples they’ll later build their base in.
“The Sith Empire” series-comprised of two miniseries-Golden Age of the Sith and Fall of the Sith Empire. Both were written by Kevin J. Anderson, who was a major Star Wars novelist and comic writer at the time, and drawn by David Carrasco.
A lot of what the series built is actually one of the few aspects of the old canon still sort of kept around-Naga Sadow’s Sith incarnation is part of the backstory of the “Old Republic” MMO game that’s still running.