Exar Kun was a Human male Jedi Knight who fell to the dark side, assumed the mantle of Dark Lord of the Sith, and waged a destructive war against the Galactic Republic and Jedi Order during the era of the Old Sith Wars. Kun was apprenticed to the respected Jedi Master Vodo-Siosk Baas, and in his days as a student he came across forbidden knowledge of the history of the Sith. Extremely confident in his own abilities and naturally quick to anger, Kun was filled with a burning curiosity regarding the Sith, and soon went on his own quest to find out as much as he could. Kun’s search led him to the planet Onderon, which had once been ruled by a Sith Lord named Freedon Nadd. With the help of a pair of Nadd’s followers, Kun came across Nadd’s tomb on Onderon’s moon Dxun, and there was confronted with the spirit of the dead Sith himself. Seeing in Kun a potential avenue to regain a physical form, Nadd directed the young Jedi to the ancient Sith homeworld of Korriban. There, Nadd conspired to grievously injure Kun, only healing the man when he accepted the dark side into his heart. Kun’s next stop was to the moon Yavin 4, a place with its own Sith history—Kun enslaved the primitive Massassi living there and compelled them to build extensive temple complexes across the forested world. Soon afterward the fully corrupted Kun banished Nadd’s spirit, having never trusted the old Sith, and in the process learned of several other nascent dark side users who had learned from Nadd.
Seeing Nadd’s other apprentices—Satal and Aleema Keto, Tetan nobles who had founded the Krath cult and conquered their native star system—as a threat, Kun set out to destroy them. On Empress Teta, Kun encountered Ulic Qel-Droma, a Jedi Knight who had lost his Master to a Krath attack and had gone undercover within the group to seek revenge. Qel-Droma had been seduced by the dark side himself, and while the two briefly dueled, their battle was ended when the spirit of long-dead Dark Lord of the Sith Marka Ragnos appeared to proclaim the two fit to call themselves Sith Lords. Exar Kun was to be the Dark Lord of the Sith, and Qel-Droma his apprentice, and together they resolved to bring a Sith golden age back to the galaxy. Allied with the remnants of the Krath and the Mandalorian warrior clans, Kun and Qel-Droma commenced a bloody war against the Republic starting in 3996 BBY. In the early stages of the conflict, Kun traveled to the Jedi library world of Ossus, murdered the Jedi Keeper of Antiquities, Odan-Urr, and took possession of a rare Sith holocron. Kun then took several prospective Jedi followers back to Yavin 4, where he used the holocron’s power to corrupt them and reshape them into his Brotherhood of the Sith. Kun sent his Brotherhood out into the galaxy to kill their Masters and wreak havoc, while he headed to the Republic capital of Coruscant to rescue Qel-Droma, who had been captured after launching an ill-advised attack on the planet.
Ever the showman, Kun burst into the Senate Hall in the middle of Qel-Droma’s trial for war crimes, murdering Supreme Chancellor Sidrona and setting Qel-Droma free. On his way out, he was confronted by his former Master, Vodo-Siosk Baas—unable to talk the Krevaaki into joining his side, Kun instead unveiled his new invention, the double-bladed lightsaber, and killed Baas in a duel. Distrustful of Aleema Keto, who had been responsible for Qel-Droma’s capture, Kun sent her to her death soon afterward, manipulating her into causing a massive supernova in the Cron Cluster that was sure to destroy the planet Ossus. Ahead of the Jedi Order’s chaotic evacuation of the doomed planet, Kun and Qel-Droma descended onto Ossus and ransacked the Great Jedi Library for its ancient treasures. While Kun made it in and out with a sizable haul, Qel-Droma got caught in combat, killed his brother, was taken prisoner and stripped of his connection to the Force. Shattered by his brother’s death, Qel-Droma switched sides and led the Jedi to Kun’s base on Yavin 4. In the ensuing battle, Kun refused to go quietly, conducting an ancient ritual to free his spirit from his mortal form so he could wreak havoc through the cosmos. He did not entirely succeed: while the ritual worked as intended and the Jedi thought him dead, Kun’s spirit was trapped on Yavin 4 for the next several millennia.
In 11 ABY, the Jedi Master Luke Skywalker established a Jedi training ground on Yavin 4, as he attempted to build a New Jedi Order after the old Order’s destruction at the hands of the Galactic Empire. After thousands of years in isolation, Kun attempted to influence Skywalker’s apprentices to do his bidding, successfully turning the young Padawan Kyp Durron to the dark side and severely wounding Skywalker. However, the rest of Skywalker’s apprentices banded together and, with the help of the long-dead shade of Vodo-Siosk Baas, banished Kun’s spirit and finally put an end to his schemes.