One of the Sith Emperor's mightiest weapons in the Great War was also one of his oldest: six ancient Sith Lords known as the Dread Masters who had served the Emperor for centuries. Meditating in unison, they could inflict crippling fear to shatter enemy armies' morale, causing Republic troop formations to abandoned their allies mid-battle, and sending entire battle fleets running blindly into hyperspace.
Slowly, the Jedi pieced together the truth of these events. They joined forces with an elite Republic Special Forces unit, who stole the security codes for the Dread Masters' heavily-guarded dreadnaught. Hijacking an Imperial Moff's personal yacht, a Republic strike team boarded the dreadnaught and, aided by Jedi Master Jaric Kaedan, captured the Dread Masters. The dreadnaught was rigged for self-destruct to cover the strike team's escape.
The Republic officially announced that the Dread Masters had been killed, even as the Jedi secretly arranged their imprisonment on Belsavis. However, the loss of the Dread Masters antagonized the Dark Council, and may have contributed to the Empire's decision to attack Alderaan.