Exar Kun was a male human Jedi Knight who fell from the light and turned to the dark side. Becoming the Dark Lord of the Sith, he led the Brotherhood of the Sith against the Jedi and the Galactic Republic during the Great Sith War.
As a young Padawan, Kun showed a strong connection to the Force. Only his master, Vodo Siosk-Baas, was able to best him in lightsaber combat. However, Kun was a rash and talented youth; qualities that resulted in considerable arrogance, a dangerous trait for a Jedi. During his training, he frequently dueled with and defeated his fellow Padawans, Sylvar and Crado, with barely controlled rage. He nearly struck down Sylvar after she clawed him in the face and insulted him by stating that he was not a Jedi, had Master Vodo-Siosk Baas not intervened. Baas defeated Kun in a sparring duel, but when Kun took a second lightsaber, he defeated Baas using Jar'Kai. The temptations of power overwhelmed Kun when he briefly viewed his Master's holocron.
While he was a Jedi Padawan, he displayed an unusual curiosity into forbidden teachings. He ignored his Master's cautions because he believed himself to be impervious to allure of the dark side. Upon completion of his training, he abandoned his master and set out in pursuit of the knowledge and power of the ancient Sith. Initially, his quest led him to the planet Onderon and then its jungle moon, Dxun, where he entered the tomb of the long-dead Freedon Nadd, another Jedi who had fallen to the dark side and became the Dark Lord of the Sith. With Kun's intrusion, Nadd's spirit awoke.
Seduced by Nadd with the promise of power through acceptance of the dark side of the Force, Kun allowed himself to become Nadd's apprentice. As Kun rapidly progressed in Sith magic, Nadd directed him to Yavin 4, where Kun acquired even greater Sith powers. He then destroyed Nadd's spirit, and set out on his own quest for galactic domination.
Kun was aware of several of Nadd's other subjects, and saw them as obstructions to his ascension to the mantle of Dark Lord. Thus, he journeyed to the Empress Teta system to eliminate the Krath Sith cult, the other darksiders under Nadd's instruction. After a brief confrontation with Krath warlord Ulic Qel-Droma, Kun and his opponent were visited by the ghost of the ancient Sith Lord Marka Ragnos—who completed their initiation into the ways of the dark side and christened Kun the new Dark Lord, with Qel-Droma as his first apprentice. Kun then partnered with Qel-Droma's Krath to form the infamous Brotherhood of the Sith, and together, they waged total war upon both the Galactic Republic and the Jedi Order which protected it.
After traveling to Ossus and killing Odan-Urr, the Jedi Keeper of Antiquities, Exar Kun stole an ancient Sith holocron in the Draethos Master's possession and used it to corrupt many Jedi Knights to his cause, forming the foundations of a Sith-inspired galactic empire. He then ordered his converts to strike down their masters in an attempt to destabilize the Jedi Order's leadership; Kun himself would confront his own former Master, Vodo-Siosk Baas, on Coruscant during the trial of Ulic Qel-Droma, where he slew both the Supreme Chancellor of the Republic and Master Baas. Kun then set Qel-Droma free, and the two Lords of the Sith resumed their campaign of galactic terror.
After instigating the destruction of the Cron star cluster, Exar Kun led his forces in a raid on the Jedi's stronghold world of Ossus, where he pillaged as much of the ancient lore of the Jedi as possible. He then retreated to his temple on the moon of Yavin 4, but was unaware that Ulic Qel-Droma had betrayed him to the Jedi, and had also led them to Yavin to stage a final offensive against him. Using an ancient Sith technique that even he did not understand, Kun separated his spirit from his body in an attempt to elude capture. However, he was defeated when Nomi Sunrider led thousands of Jedi in the creation of a wall of light; the power of which destroyed the landscape of Yavin 4, and trapped the Dark Lord's spirit within the walls of his temple.
He, his brotherhood and the surviving cultists are not aligned with the current Sith Empire due to the fact that the Ancient Sith's cooperation and support of Exar Kun is not shared with the modern Sith Empire. Many surviving Sith who followed Exar Kun during the Great Sith War went on to fight for the Sith Empire led by Darth Revan during the Jedi Civil War that occurred around 40 years later. Darth Sion himself fought as a Sith Marauder for Kun's Empire.