Upon slaying Malgus' lieutenant, Darth Serevin, the combined forces attacked Malgus' mobile space station and destroyed his armies, before defeating Malgus himself in his throne room.
After the Sith Empire had discovered that the Adegan crystal on Ilum possessed innate stealth properties, Imperial officers - particularly Grand Moff Ilyan Regus, the successor to the late Rycus Kilran - dreamed of using the planet's resources to create a fleet of stealth warships for the Imperial Armada. In 3640 BBY, Regus and Dark Council member Darth Arho led Imperial forces in an invasion of Ilum codenamed "Operation Dark Ice". In response, the Republic sent a considerable army to stop them, under the leadership of Supreme Commander Rans and Jedi Master Jaric Kaedan.
The Empire made tremendous gains in the initial phases of the battle, slaying Master Kaedan and Admiral Shai. Forces under the personal command of Darth Arho were also able to capture Supreme Commander Rans during a sneak attack on the Republic base camp in the eastern ice fields. As the Republic commanders fretted on what to do next, they were contacted by an unlikely ally: Darth Malgus, a man known for his desire to destroy the Republic, but willing to accept the reality that the Republic was the victor, and it required Sith of a more "progressive" mindset in order to save the Empire.
During the battle, the Republic's Talz commandos carried out an operation codenamed "Shatterstorm." Transmissions mentioning the operation were intercepted by the Operative Che'nash of the Shrikaansi Reconnaissance Company, who was unable to proceed on her own and hired in Imperial to infiltrate the Talz camp and extract more information from their communications grid.
Malgus revealed that he had secretly been in contact with Rans, feeding him information on how best to deal with Darth Arho, who Malgus considered a "thug" despite being on the Dark Council. He informed the commanders, and the Republic and Jedi heroes answering the call to arms on Ilum, that Rans was being held at a prison camp to the west of their base, guarded by sensitive sensor towers that would trigger alarms if anyone attempted to enter the base. Malgus provided the codes to deactivate the sensor towers, allowing the Republic forces to free Rans and some of his soldiers.
With Malgus' aid, the Republic forces tracked down Arho himself to the ruined Ilum Temple. Though Arho was incredibly powerful with the Force, he was no match for his opponents.
Despite Arho's death, the Empire had gathered a vast resource of Adegan crystals, achieving the goal of their operation. However, Malgus seized the fleet carrying the crystals, much to the consternation of Grand Moff Regus. Shortly afterwards, Malgus transmitted a message across the entire galactic HoloNet. In it, he condemned the apparent apathy (and later disappearance) of the Sith Emperor, and the infighting and xenophobic mentality of the Dark Council and the Sith as a whole. Thus, he declared a "new Empire", open to all warriors willing to follow him, and decreed that the Dark Council be dissolved. Malgus' declaration blindsided both the Republic and the Empire, both of whom summoned emergency councils of war to respond to the threat. With such a wealth of Adegan crystals at his disposal, Malgus could create a stealth armada that would make him virtually invincible, and perfectly capable of carrying out his designs for a new war of conquest.
Making the first move, Malgus dispatched Serevin to lead an invasion of Ilum to destroy the Republic and Imperial forces there. Both sides believed that capturing a stealth fighter with the command codes would allow them to locate Malgus himself. Cole Cantarus of the Corellian Security Force and General Hesker of the Imperial Guard, both veterans of the battle for Corellia, flew their respective strike teams to the front lines to confront Serevin and his forces at Fort Barrow. Serevin faced his attackers accompanied by Voss Mystics and Commandos he had recruited during his period as a diplomat in Voss-Ka. Serevin was struck down, and the combined forces took control of his stealth fighter. The ship's computer provided coordinates to the Emperor's Fortress, commandeered by Malgus after the Voice of the Emperor was defeated by the Hero of Tython. Rather than waste time translating the coordinates and taking their own ships, the combined forces took the stealth ship and landed onboard Malgus' space station.
The army at Malgus' command on the space station included a wide variety of troops: defecting Sith and Imperial soldiers, Trandoshan warriors, Mandalorian mercenaries, Anomid scientists, and extermination droids from the Foundry, a Rakata space station captured by an Imperial strike team at Malgus' direction. The Anomids, belonging to a group known as the Schism Collective, had been behind the successful creation of Malgus' stealth armada, and also outfitted the station and the troops with salvaged Rakata technology from the Foundry. Fighting their way through the station's halls, they found Malgus in his throne room. As the enemy forces outside began to press the attack, Malgus activated the station's self-destruct sequence, which would incinerate the station and the attacking fleets, before leaping down from his throne to face his attackers.
Decades of fighting on the front lines made Malgus a formidable opponent, but his attackers managed to gain the upper hand. Seizing an opening as Malgus' resolve faltered, the attackers used Force powers and pulse grenades to force him off the central platform of the station. The self-proclaimed Emperor fell from the platform and down the reactor shaft, presumably to his death. His "new Empire" would die while he disappeared.
After Malgus' defeat, Ilum was firmly in the hands of the Republic, and the Empire had managed to reclaim the Adegan crystals it had taken from the planet. But Malgus' rebellion was a major blow to the Imperial war effort; combined with the infighting amongst the Sith and the reversals of fortune on the battlefield, the Empire was brought to the brink of collapse by the following year, leaving its surviving leaders to hope for a miracle that would prevent their downfall.
Two years after the Battle of Ilum, a seemingly insignificant world located on the border between the Republic and Hutt Space came to their attention as a possible solution to the Empire's misfortune: Makeb.
- The Art and Making of Star Wars: The Old Republic
- Star Wars: The Old Republic Explorer's Guide
- Star Wars: The Old Republic Encyclopedia
- Nexus of Power