Darth Wyyrlok was a Chagrian Lord and foremost loremaster of the One Sith. He was afforded a title of hereditary esteem in his capacity as the most trusted servant of the Dark Lord Darth Krayt, only to whom Wyyrlok was second in knowledge and power. When the Dark Lord usurped the Imperial throne and installed himself as Galactic Emperor, Darth Wyyrlok served as steward of the new Sith Empire, when so required, in his Lord's stead.
Over seven years, Krayt continued to establish his empire with Wyyrlok at his side. Simultaneously, the new Emperor's body continued to deteriorate, due to the presence of Yorik-Kul in his body. Wyyrlok sought remedies to the ailment, but was unable to find a cure for his Master. Krayt attempted to bring Cade Skywalker, who possessed unique healing abilities, into his Sith Order, but failed to do so. Although Krayt remained optimistic about his condition, Wyyrlok was not satisfied, and he set off on a perilous journey into the Deep Core to find a cure for his ailing Master. Unable to find one, Wyyrlok later killed his Master on Had Abbadon, after Krayt had been severely injured in a battle with Imperial Knights and criminals led by Skywalker. However, Wyyrlok was the only member of the One Sith that knew of Krayt's demise; he collected and interred the Dark Lord's body in a stasis chamber within XoXaan's Temple on Korriban. While pretending that Krayt had resumed one of his many periods of regenerative hibernation, Wyyrlok traveled to Coruscant where he assumed the Dark Lord's position as Galactic Emperor, and continued implementation of Krayt's dream—to initiate every living being of the galaxy into the One Sith Order. However, when Krayt was resurrected, he punished Wyyrlok for his treachery. After Krayt led an attack on the Temple of the Sith on Coruscant, he and Wyyrlok dueled, ending in Wyyrlok's death at the hands of his former master.