After his corruption, Revan took ships beyond the known galaxy in search of an ancient artifact, a megastructure known as the Star Forge. For several years prior to the war, Revan and Malak had been researching artifacts in reference to this powerful Star Forge. They discovered that it was a massive space station with the ability to produce an almost limitless number of ships, accomplished by siphoning matter and energy from a nearby star as well as feeding on the dark side energies imbued within it. Revan and Malak, now beholden to the dark side, created a massive fleet to destroy the Old Republic and the Jedi, reforming the ex-Republic forces who followed them into a militaristic empire.

The reinvigorated Sith won battle after battle, until a young Jedi named Bastila Shan entered the fray with her Battle Meditation. This rare and powerful Force ability increased the combat effectiveness of the Old Republic's forces and sapped the will of enemy combatants. Even with this power, however, the Old Republic's forces knew that they could not hold out forever against the Sith. In a desperate attempt to end the war, the Old Republic set a trap for Revan and Malak, during which Bastila and a team of Jedi boarded Revan's flagship and managed to corner the Dark Lord on the bridge. Revan most likely would have defeated Bastila and the remnants of her Jedi strike team if he was given the chance. However, Malak, likely knowing he could not defeat Revan in personal combat, took this opportunity to kill both Revan and Bastila with one swift stroke. He fired upon the flagship from a nearby battle cruiser but failed to kill either his master or Bastila. Revan, nearly dead, was partly healed by Bastila Shan, creating a Force bond between the two. His mind was reprogrammed by the Jedi Council to hide the Dark Lord's old identity, while enabling the Jedi to use Revan's subconscious memories to locate the Star Forge and destroy it.

** Taken From: http://en.wikipedia.org/wiki/Darth_Revan