Lord Ulthar was a Sith Lord living during the time of the founding of Korribanos. He was one of the Sith Lords serving under Lord Adas in the wars with the Jedi and the Galactic Republic in the Old Galaxy.
Although conceived of by Darth Azeroth, it was Lord Ulthar who carried out the Rules of Two for the first time. He is also the one who assisted Azeroth and Aeris in their plan to place themselves and an army into carbon freeze. Without Lord Ulthar, Azeroth and Aeris' plan would never have made it past the concept phase. He took what Azeroth taught him and passed it down to his Apprentice. His legacy would survive for over four millennia, as Azeroth and Aeris would discover when they came out of hibernation and met with the Sith Lord of Ulthar's line in 14 NE.