Attack on Korriban

"Do not underestimate the Sith. Though quaint, they will do whatever it takes to survive."

- Immortal Emperor Valkorion

During the fifth year of the renewed warfare between the Galactic Republic and the Sith Empire, the Eternal Empire made an attack on the planet Korriban. It was part of a series of preliminary raids by the Empire, which up to that point had been concealed to the rest of the galaxy, as a test of the strength of the border worlds of the Republic and the Sith. Korriban was a prime target for the Eternal Empire, due to the presence of the Sith Academy for the training of Acolytes. The twin princes Arcann and Thexan led the assault, where they faced fierce resistance. Arcann was critically injured during the battle, suffering severe burns and the loss of an arm and an eye. However, the following day he was on his feet again and executed the Sith commander Darth Atroxa. Despite the efforts of the Sith, the Knights of Zakuul breached the Sith Academy during the fighting.

The attack was a tremendous defeat for the Sith, while the Eternal Empire only suffered moderate casualties. Following the battle, the forces of the Eternal Empire withdrew, eventually heading back to their throne-world of Zakuul. Shortly afterwards, the Republic and the Sith Empire joined together to find the source of these unknown aggressors.

Prelude
"Look around you. Zakuul is poised to become the greatest civilization in the history of the galaxy. I have forged this empire to surmount all of my previous works. To span eternity."

- Immortal Emperor Valkorion

Located in the Esstran sector of the Outer Rim Territories, Korriban was the homeworld of the Sith species as well as the subsequent Sith Order, though the Great Hyperspace War against the Galactic Republic saw the ruin of the Sith Empire. Millennia later, Vitiate, Emperor of the reconstituted Sith Empire, grew weary of the restraints placed upon him by the Sith and his Empire. Freeing himself, he split his consciousness into a new body&mdash;Valkorion, and set to work forging a new empire from the swamps of the planet Zakuul; the Eternal Empire. The new realm became immensely powerful, all the while remaining concealed from the rest of the galaxy in Wild Space.

Centuries later, the Sith Empire returned to the known galaxy, initiating the Great Galactic War against the unsuspecting Republic. Early in the war, the Sith stormed Korriban, overwhelming the few Republic defenders. The abandoned Sith Academy, previously used three centuries earlier during the Jedi Civil War, once again saw use, turning out only the most powerful Acolytes.

Although the Treaty of Coruscant was signed decades later between the two opposing sides, the resulting tension between the Empire and Republic eventually overflowed, spilling into yet another war. In 3637 BBY, the fifth year of the conflict, Valkorion's twin sons, Princes Arcann and Thexan, proposed a plan to test the capabilities of the Republic and Empire in combat. Valkorion agreed, granting Thexan control of the Eternal Fleet &mdash;an armada operated by sentient droids, though he ordered that Arcann remain on Zakuul. However, Arcann disobeyed his father's wishes and led the assault alongside his brother regardless. They fought on many worlds, before finally striking at Korriban.

The attack
"Acceptable casualties, on our side. Darth Atroxa remains a threat, but I'm confident she will be neutralized. With her downfall, victory will be ours."

- Thexan

The forces of the Eternal Empire, composed of starfighters, shuttles and at least three battleships descended on Korriban, facing fierce resistance from the Sith's own naval forces. The latter was comprised 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 on the Sith Academy on foot. They clashed with the Sith under the command of Darth Atroxa in the Valley of the Dark Lords, whose forces were composed of Sith Warriors as well as Acolytes and Overseers from the Academy. During the battle, Arcann was grievously wounded. As several Knights stood guard, the surgeons operated upon him. The prince was later taken away from the battle to an encampment, where he thrashed in pain for hours before finally succumbing to sleep. There, Arcann was further performed upon; by the end of the ordeal, he had been fitted with a replacement cybernetic arm and eye&mdash;the latter as a part of a durasteel mask made to protect the prince's burnt face and to intimidate his foes.

While Arcann was incapacitated, Thexan updated Valkorion on the status of the battle via holocom. The emperor did not express concern over his son's injuries, remarking that Arcann had "paid the price for defiance" in leaving Zakuul. The following day, Arcann was on his feet again and led a renewed assault during an intense sandstorm. Darth Atroxa, the Sith commander, met her end by the prince's lightsaber after he used his cybernetic arm to twist her wrist. This disarmed her, allowing Arcann to land the finishing blow.

Despite Sith resistance, the Eternal Empire stormed the Sith Academy, where Sith Acolyte Veeroa Denz decapitated her Overseer, using the confusion to escape Korriban and the Sith.

Aftermath
"I was training on Korriban when the Eternal Empire's forces attacked. I learned afterwards that Arcann and his brother led the invasion."

- Veeroa Denz

The attack was a crushing defeat for the Sith. Not only had they suffered heavy casualties, but considerable damage had been done to a prominent academy for the training of prospective Sith. The Eternal Empire also sustained sizable losses during the battle, though not as extensive as those of the Sith. As a result of the attack, many Acolytes fled and settled down to live their lives without interference from the Sith or the Eternal Empire, such as Veeroa Denz on Nar Shaddaa. When the raids were concluded, the forces of the Eternal Empire returned to Zakuul. A joint Republic–Sith taskforce led by Darth Marr was dispatched to locate the source of these unknown assailants. During the ensuing skirmish with the Eternal Fleet, forces of the Sith Empire identified the starships of the Fleet as belonging to the same faction that had earlier attacked Korriban.

Behind the scenes
The attack on Korriban made its first appearance in the cinematic trailer Sacrifice, released on June 15, 2015. The trailer promoted the then-upcoming expansion for the massively multiplayer online roleplaying game Star Wars: The Old Republic, titled Knights of the Fallen Empire. The short story Brothers, published on The Old Republic official website a month after Sacrifice, detailed Thexan's perspective on the battle. The expansion itself was released later that year, where Veeroa Denz, an Acolyte who had been training in the Sith Academy during the attack, gave further insight into the battle in the Star Fortress quest line. It also received a brief mention within the in-game codex entry for Arcann, as well as during Chapter I: The Hunt. The attack served to establish how Arcann sustained his injuries and received his cybernetic replacements, which led to his change of personality to a darker tone.