During the fifth year of the Galactic Republic's and the Sith Empire's revived conflict, the Eternal Empire launched an assault upon the planet of Korriban. This assault was one of a sequence of initial attacks conducted by the Empire, which, until this point, had been hidden from the rest of the galaxy, to gauge the strength of the Republic and Sith's border territories.
Korriban was a key target for the Eternal Empire, due to the Sith Academy being located there, which was used for the training of Acolytes. The princes, who were twins, Arcann and Thexan, spearheaded the attack with the force-sensitive Knights of Zakuul, encountering significant resistance. Arcann suffered critical wounds during the confrontation, including severe burns and the loss of a limb and an eye. However, on the following day, he was back on his feet and executed the Sith commander, Darth Atroxa. In spite of the Sith's best efforts, the Knights of Zakuul successfully penetrated the Sith Academy during the battle.
The assault resulted in a major defeat for the Sith, whereas the Eternal Empire's losses were relatively minor. Following the battle, the Eternal Empire's forces retreated, eventually returning to their throne-world, Zakuul. Soon after, the Republic and the Sith Empire formed an alliance to discover the origin of these mysterious aggressors.
Korriban, situated in the Esstran sector of the Outer Rim Territories, was the original homeworld of both the Sith species and the Sith Order, although the Great Hyperspace War against the Galactic Republic led to the downfall of the Sith Empire. Millennia later, Vitiate, the Emperor of the reconstituted Sith Empire, grew weary of the restrictions imposed on him by the Sith and his Empire. After freeing himself, he transferred his essence into a new body, a man named Valkorion. He then began constructing a new empire from the swamps of the planet Zakuul, which evolved into the Eternal Empire. This new empire became incredibly powerful, all while remaining hidden from the rest of the galaxy in Wild Space.
Centuries later, the Sith Empire reappeared in the known galaxy, initiating the Great Galactic War against the unsuspecting Republic. Early in the war, the Sith overran Korriban, overwhelming the small number of Republic defenders. The abandoned Sith Academy, which had been used three centuries prior during the Jedi Civil War, was once again put into service, training only the most formidable Acolytes.
Although the Treaty of Coruscant was signed decades later between the two opposing factions, the resulting tension between the Empire and Republic eventually escalated, leading to another war. Around 3636 BBY, during the fifth year of the conflict, Valkorion's twin sons, Princes Arcann and Thexan, proposed a strategy to evaluate the combat capabilities of the Republic and Empire. Valkorion approved, granting Thexan command of the Eternal Fleet—an armada controlled by sentient droids—but ordered Arcann to remain on Zakuul. However, Arcann defied his father's orders and led the attack alongside his brother regardless. They engaged in combat on numerous worlds before finally targeting Korriban.
The Eternal Empire's forces, consisting of starfighters, shuttles, at least two battleships, and one warship, descended upon Korriban, encountering strong resistance from the Sith's own naval forces. The latter was composed of Mark VI Supremacy-class starfighters and Fury-class Imperial interceptors. Led by Arcann and Thexan, the Force-sensitive Knights of Zakuul and several droids advanced toward the Sith Academy on foot.
They clashed with the Sith, commanded by Darth Atroxa, in the Valley of the Dark Lords. The Sith forces included Sith Warriors, as well as Acolytes and Overseers from the Academy. During the battle, Arcann sustained severe injuries. While several Knights provided protection, medics attended to him. The prince was subsequently removed from the battlefield to a camp, where he writhed in agony for hours before finally falling asleep. There, Arcann underwent further medical procedures; by the end, he had been fitted with a replacement cybernetic arm and eye—the latter being part of a durasteel mask designed to protect the prince's burned face and instill fear in his enemies.
While Arcann was unable to fight, Thexan informed Valkorion about the battle's progress via holocom. The emperor showed no concern for his son's injuries, commenting that Arcann had "paid the price for defiance" by leaving Zakuul. The following day, Arcann was back on his feet and led a renewed attack during a fierce sandstorm. Darth Atroxa, the Sith commander, met her death at the prince's lightsaber after he used his cybernetic arm to twist her wrist and disarm her.
Darth Hexid was among the Sith who participated in the defense, fighting through the invaders until she was incapacitated by an explosion. Despite the Sith's resistance, the Eternal Empire stormed the Sith Academy, where Sith Acolyte Veeroa Denz decapitated her Overseer, using the chaos to escape Korriban and the Sith.
The attack was a devastating defeat for the Sith. They not only suffered significant casualties but also sustained considerable damage to a prominent academy used for training future Sith. The Eternal Empire also incurred substantial losses during the battle, although they were not as extensive as those of the Sith.
As a consequence of the attack, many Acolytes fled and chose to live their lives without interference from the Sith or the Eternal Empire, such as Veeroa Denz on Nar Shaddaa, while Darth Hexid, despising the weakness of the Sith who submitted to the Eternal Empire, embarked on her own personal vendetta against Zakuul. After the attacks concluded, the Eternal Empire's forces returned to Zakuul. A joint Republic-Sith task force, led by Darth Marr, was dispatched to locate the origin of these unknown attackers. During the subsequent skirmish with the Eternal Fleet, Sith Empire forces identified the starships of the Fleet as belonging to the same faction that had previously attacked Korriban.
The attack on Korriban was first shown in the cinematic trailer titled Sacrifice, which was released on June 15, 2015. The trailer promoted the then-upcoming expansion for the massively multiplayer online roleplaying game Star Wars: The Old Republic, called Knights of the Fallen Empire. The attack served to explain how Arcann sustained his injuries and received his cybernetic replacements, which led to his personality becoming darker.