Exar Kun, Dark Lord of the Sith is a fictional character in the Star Wars Expanded Universe. His first appearance was in Kevin J. Anderson's Jedi Academy trilogy.
Exar Kun wields exceptional power in the Force (the ground "trembled with his footsteps," as a character in Knights of the Old Republic remarks). He was extremely arrogant, an unbecoming and dangerous quality in a Jedi, but one perfect for a Sith. The temptation of power overwhelms Kun when he briefly viewed a rare and forbidden Sith holocron, which belonged to his Master, Vodo-Siosk Baas. Master Baas was greatly displeased upon learning of his pupil's meddling; the knowledge of the dark side was dangerous in the hands of such an unpredictable mind. Indeed, it was becoming apparent to Baas that Kun's pride and ambition were beginning to consume him; Kun was convinced that he could master the dark side while remaining uncorrupted and go on to teach the Jedi Order his new discoveries. Baas tried to save his pupil, warning him to heed only Baas' lessons and to avoid the snares of the dark side, but his pupil disregarded his instructions. After defeating his master in combat, Kun followed his curiosity and traveled to the war-torn planet of Onderon. There, he learned of a tomb that housed the mortal remains of an ancient Sith Lord, Freedon Nadd; the Jedi had sealed it in Mandalorian iron and sequestered it in a remote section of the demon-moon Dxun. These barriers were insufficient to deter the young Jedi, however, and he made his way to the dark glade. There, the spirit of the long-deceased Sith Lord appeared to Kun, and, using the young Jedi's curiosity and ambition to his advantage, the ancient Sith instructed Kun to continue his quest to the infamous Sith tomb-world of Korriban.
On Korriban, Kun delved deep into it's mausoleums that encrusted its surface. The spirit of Freedon Nadd, and those of the other Sith Lords, decided to test the wayward Jedi. He was cut off from the force and trapped in a cave in. The Sith offered Kun escape by fully embracing the Dark Side. He did not, but corrupted himself in anger and hate anyway in a successful escape attempt.
Before fully embracing the dark side, Exar Kun was directed to the jungle moon of Yavin IV, one of the few remaining Sith outposts. Even after Freedon Nadd ransacked the treasures deposited there by Naga Sadow, there was much to attract Kun. There he encountered the primitive remnants of the experiment of the Sith Lord Naga Sadow, the fearsome Massassi race. Denied the powers of the light, but at the same time refusing to entirely accept the dark side, Kun was easily captured by these natives. As Kun, on the verge of death, was being crushed by a giant creature of alchemical origins, Freedon Nadd proceeded to interrupt another of his student's predicaments. In what would be Kun's final moments before being totally corrupted by the dark side, Nadd instructed his pupil that his only escape was to once again tap into the darkness within him. Taking the Sith amulet being displayed at his Massassi sacrifice in hand, Kun embraced its dark nature with absolute conviction. Transformed into an agent of darkness, he broke free from his captors, slaughtering them without hesitation. While Nadd was pleased by Kun's newfound strength, the Dark Jedi became fed up with the ancient Sith's tests and meddling. He then destroyed Freedon Nadd, who cursed him as he died and warned his other proteges of Kun's actions.
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Links: Wikipedia - Exar Kun Wookieepedia - Exar Kun Star Wars: Databank: Exar Kun