This particular person served as a Dark Jedi within Revan's Sith Empire. After Darth Malak took command of the Empire, this Dark Jedi pledged loyalty to the new Dark Lord and was regarded by Malak as one of his top three most formidable adherents.
This Dark Jedi utilized the Jar'Kai style of combat.
This Dark Jedi is only seen in the non-canon dark side conclusion of Star Wars: Knights of the Old Republic, where Revan and Bastila Shan eliminate him. Since this event is not part of the official Star Wars timeline, it is believed that he survived the Jedi Civil War.