Ergast was a Sith Lord of the Old Sith Empire. He discovered the technique of Force walk and was the first Sith Lord that was buried on Dromund Kaas. He was the gatekeeper of his own Sith holocron, a simulacrum that was referred to as the "Servant of Ergast." In life, he wielded a purple-bladed lightsaber.
Lord Ergast was one of the first Sith who was buried on Dromund Kaas, and the first Sith Lord, having been alive thousands of years before the Cold War. Following the defeat of the Empire's forces during the Great Hyperspace War at the hands of the Galactic Republic and the Jedi Order, the Sith Emperor's fleet fled from the advancing forces to a world where they could be safe from the forces of the Republic and the Jedi. In 4980 BBY, a scout ship from the fleet re-discovered the planet of Dromund Kaas. Subsequently, other Sith in the fleet settled down on the world and proceeded to build a new city, which would serve as the capital of a new and restored Sith Empire. During this time, Ergast's tomb was rediscovered, and the larger Dark Temple built over it.
During the Cold War, Ergast's ghost resided in his tomb within the Dark Temple on Dromund Kaas, where he was visited by Lord Kallig, who desired to learn the Force walk to defeat Darth Thanaton. Ergast instructed the young Sith Inquisitor to drink a cup of poison before them, which would not kill them. In some nether realm, Ergast taught all he knew of the Force walk, including the fact that more power could be drawn from an unwilling ghost than a willing one. As soon as Kallig regained consciousness, Ergast added that he would not go with them willingly as he was absorbed by the Sith Lord. Kallig later went on to absorbing the ghosts of Darth Andru, Kalatosh Zavros, and Horak-mul. But drawing on their power in a confrontation with Thanaton proved too much to handle. Ergast then admitted that he forgotten to mention that the Force walk could be damaging if more than one ghost were to be bound.
Kallig soon sought to find a solution to this sickness before the power became too much or the Sith ghosts threatened to drive them insane. First, on Belsavis, Kallig found a Rakata machine that altered the Sith body to hold the four ghosts' power. But as the Inquisitor journeyed to Voss, Ergast mocked Kallig for thinking that they could be silenced by some magic ritual. As Kallig undertook a dreamwalking experience, Ergast took the form of Urtel Moren and attempted to kill Kallig, but was defeated. Later, with the aid of a Gormak Force shaman named Hadrik, using a dream rock exposed Ergast's true form so he could truly be subdued.
After Kallig defeated Thanaton and gained his seat on the Dark Council, the ghosts pleaded to be freed but Kallig refused. Cursing the Dark Lord, Ergast vowed they would regain their freedom, but was cut off in mid-sentence as Kallig bound them all.
- Star Wars: The Old Republic (First appearance) (Appears as a ghost or a spirit)