This specific figure was a Dark Jedi serving within Revan's Sith Empire. After Darth Malak took command of the Empire, this Dark Jedi pledged their loyalty to the new Dark Lord, and Malak regarded them as one of his top three strongest adherents.
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. Given that this event is not part of the official Star Wars timeline, it's believed that he survived the Jedi Civil War.