The Jedi Civil War
Shortly after the Great Sith War, the remnants of the Mandalorians sensed weakness in the Republic and decided to strike. A long and bloody war followed between the Jedi and the Mandolorians, but in the end the Jedi prevailed. Two Jedi named Revan and Malak led a Republic fleet to the outer rim and successfully wiped out the Mandalorians. At the end of the war, Revan and Malak were revered as heroes.
Little did the Republic know that their Jedi heroes were destined to become two of the most brutal Sith Lords ever known. While in deep space, the two Jedi landed on Korribann and uncovered many secrets of the Sith. Corrupted by their newfound knowledge and the brutality of the war they had just won, the two Jedi renounced the Order, became Sith and changed their names to Darth Revan and Darth Malak -- master and apprentice.
The two Sith Lords had the loyalty of Jedi and Republic forces that they led to destroy the Mandalorians. They took their new Sith fleet and waged a two-year war on the Republic's outer-rim territories.
Eventually, power hungry Darth Malak tried to destroy both his master Revan and the Jedi. His plan failed, but it did allow the Jedi to capture Revan alive.
The Jedi wiped the Sith master's memory, brainwashing and retraining Revan as a Jedi. In a most uncharacteristic move, the Jedi Council approved the use of Revan as a pawn to find the fugitive Sith Lord Darth Malak.
Eventually Revan tracked Malak to Star Forge and defeated him in a lightsaber duel. After that Revan disappeared. The Sith were once again vanquished.