The Battle of Yavin IV represented a climactic confrontation during the Great Sith War, occurring in 3996 BBY. This battle brought about the end of Sith Lord Exar Kun and the near extinction of the Massassi species residing on Yavin 4. After Ulic Qel-Droma, a former Jedi Knight and Sith Lord, switched allegiances, the Jedi Order and elements from the Republic Rim Fleet descended upon Yavin IV, which served as the primary base of operations for the Brotherhood of the Sith. Kun, in an attempt to transfer his consciousness into the cosmos, performed a dark side Sith ritual, draining the life force of numerous Massassi. However, this ritual clashed with a wall of light energy field created by the participating Jedi, resulting in a devastating firestorm that ravaged the surface of Yavin IV and obliterated much of its plant and animal life. While Kun's spirit survived the conflagration, it was confined within the Massassi Temples.
The galactic-spanning conquest orchestrated by Exar Kun had resulted in the deaths of countless individuals throughout the duration of the Great Sith War. With the ultimate destruction of the primary Jedi training facility located on Ossus, the Order's period of perceived invulnerability had definitively come to an end. The necessity for a swift and decisive resolution had become increasingly crucial.
However, the Galactic Republic experienced an unexpected stroke of good fortune when Ulic Qel-Droma, a former Sith Lord, renounced the dark side following his murder of his own brother, while imprisoned beneath a Wall of light. Ulic confronted the ramifications of his actions psychologically, recognizing the imperative to reverse the devastation he had caused. By revealing the location of Exar Kun's main headquarters—the fourth moon orbiting the planet Yavin—the Jedi leaders, under the guidance of Jedi Master Thon and Nomi Sunrider, swiftly mobilized the Republic Rim Fleet to bring the fight directly to Kun.
Thousands of Jedi Knights from across the galaxy responded to the call, converging upon the jungle moon in a vast armada of warships that darkened the skies. Upon observing the arriving fleet, Kun immediately recognized Qel-Droma's betrayal and pledged to continue his struggle against the light by focusing on survival. Limited ground skirmishes took place between Republic soldiers and Kun's Massassi warriors. Unable to overcome the Rim Fleet's orbital blockade, Kun gathered a large portion of the Massassi population on Yavin IV within the Temple of Exar Kun for a final ritual. Drawing upon ancient Sith texts, he siphoned the life essence of the Massassi and channeled the energy into a Sith ritual aimed at separating his spirit from his physical form. For a brief moment, it appeared that Kun would be able to roam the cosmos without end.
In response, the assembled Jedi generated a powerful wall of Light Side energy with the intention of purging Exar Kun's corrupting presence from the galaxy and imprisoning his spirit. However, the wall unexpectedly collided and interacted with the immense surge of dark side energy emanating from Yavin, triggering a planetary chain reaction that unleashed an unprecedented firestorm, completely consuming all traces of Kun and the Massassi.
As the resulting inferno proved uncontrollable, the Jedi forces reluctantly withdrew from Yavin's orbit, while the fires continued to rage unchecked. Nomi, Ulic, Thon, and the remaining Jedi were deeply affected by the events they had witnessed, contemplating the ultimate consequences of their actions.
Two years later, Qel-Droma would eventually understand the true cost. His newly named vessel, Cay's Dream, landed on Yavin 4, revealing a devastated world and ecosystem devoid of any signs of life. No trace of Exar Kun was discovered, and Qel-Droma departed. However, within the shadows of the Massassi temple, Exar Kun's spirit issued desperate pleas for release from the darkness, condemned to the hell he had created for himself, but Ulic, unable to sense the Force, remained unaware. Over time, Yavin IV's ecosystem slowly recovered and regenerated, eventually supporting plant and animal life once more.
Some Massassi on Yavin IV, including Kalgrath, also managed to survive the firestorm. Over the centuries, their numbers on Yavin IV increased. In 3756 BBY, these Massassi were encountered by Barel Ovair, a covert spy for the True Sith, and Eison Gynt, a Jedi Padawan who were on a mission to seemingly eliminate the dark side energies emanating from the tomb of the Sith Lord Naga Sadow from the Great Hyperspace War. These Massassi attacked the two offworlders and drove them deeper into the tombs where they encountered the Sith Lord's lingering energies. Seven years later, Gynt emerged wearing Massassi warrior robes and possessed by Sadow's dark side spirit. Gynt attacked master Ovair in broad daylight in Coruscant but was bested by Ovair. In addition, there were Massassi living on other worlds including Korriban and Tund.
Nearly four millennia later in 1 ABY, Professor Walter Emanus led an archaeological expedition from Corellia University. However, during their excavations, they uncovered the catacombs beneath the Dark Lord's temple and reawakened his spirit. He possessed them and made them into the Cult of Exar Kun, which was devoted to his worship. Under his influence, some of the cultists traveled to the Corellia University research outpost on Talus to recruit more students and researchers to their ranks. The cult based itself around the catacombs, but was destroyed when Professor Cornelius Wagglehorn, a former colleague of Emanus, hired a group of spacers to drive them out. Kun himself battled the spacers, but due to his spirit form, was easily defeated and silenced once more.
In 11 ABY, Kun's temples became the location of Luke Skywalker's Jedi Praxeum, a training academy where he initiated the reformation of the Jedi Order. The Jedi trainees' exercises in the Force awakened Kun who sought to subvert the Jedi training. As a spirit, Kun appeared to the young Jedi apprentice Gantoris. The inexperienced apprentice was unable to avoid being corrupted and, when he started to resist, Kun killed him, burned from inside. After destroying his first Jedi in thousands of years, Kun's insatiable appetite for spreading his dark wrath escalated. He was able to successfully corrupt and partly possess the young and extraordinarily talented Kyp Durron.
In addition, he sundered Skywalker's spirit from his body, demoralizing his Jedi apprentices. Kun's spirit was eventually roused from hiding and banished into Chaos by the combined efforts of the Jedi apprentices, Luke's spirit, and the long-dead spirit of Kun's Master, Vodo-Siosk Baas. After Kun's shade was vanquished, Corran Horn destroyed Kun's temple with proton torpedoes, thus eliminating his dark taint.