The infamous Sith Lord Exar Kun had no relations with the true Sith Empire, but his fall to the Dark Side set the stage for the Mandalorian Wars and the eventual fall of Revan and Malak
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Exar Kun was a Jedi Knight with considerable skill with a lightsaber. Thirsting for knowledge of the force, he disregarded his master's counsel and came under the tutelage of the ancient Sith Lord Freedon Nadd.