The conflict known as the Battle of Ilum, also called Operation Dark Ice, occurred in the opening stages of the Galactic War between the Galactic Republic and the Sith Empire. It started initially as a struggle for control of the Adegan crystals on the isolated world of Ilum between the Empire and the Republic. However, Darth Malgus took control of the situation, proclaimed himself Emperor, and summoned other Sith to join his "New Empire." Consequently, both the Republic and the Sith Empire briefly put their conflict aside to deal with this emerging threat.
After eliminating Malgus' subordinate, Darth Serevin, the allied forces assaulted Malgus' mobile space station, destroying his armies, and ultimately defeating Malgus himself within his throne room.
Following the Sith Empire's discovery that the Adegan crystal found on Ilum possessed inherent stealth capabilities, Imperial officers—notably Grand Moff Ilyan Regus, who succeeded the deceased Rycus Kilran—envisioned utilizing the planet's resources to construct a fleet of stealth-capable warships for the Imperial Armada. In 3640 BBY, Regus and Dark Council member Darth Arho spearheaded Imperial forces in an invasion of Ilum, designated "Operation Dark Ice." In response, the Republic dispatched a substantial army, commanded by Supreme Commander Rans and Jedi Master Jaric Kaedan, to thwart them.
The Empire achieved considerable success during the initial phases of the battle, eliminating Master Kaedan and Admiral Shai. Forces under the direct command of Darth Arho also managed to capture Supreme Commander Rans during a surprise assault on the Republic's base camp in the eastern ice plains. As the Republic commanders deliberated on their next course of action, they received communication from an unexpected ally: Darth Malgus, 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 conflict, the Republic's Talz commandos executed an operation known as "Shatterstorm." Transmissions regarding this operation were intercepted by Operative Che'nash of the Shrikaansi Reconnaissance Company, who, unable to proceed alone, hired an Imperial agent to infiltrate the Talz camp and extract further intelligence from their communications network.
Malgus disclosed that he had been secretly communicating with Rans, providing him with information on how to best counter Darth Arho, whom Malgus considered a "thug" despite his position 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 west of their base, guarded by sensitive sensor towers that would trigger alarms upon any attempted entry. Malgus supplied the codes necessary to deactivate the sensor towers, enabling the Republic forces to liberate Rans and some of his soldiers.
With Malgus' assistance, the Republic forces located Arho himself at the ruined Ilum Temple. Despite Arho's immense power in the Force, he was ultimately defeated by his adversaries.
Despite Arho's demise, the Empire had amassed a substantial quantity of Adegan crystals, fulfilling the objective of their operation. However, Malgus seized control of the fleet transporting the crystals, much to the dismay of Grand Moff Regus. Shortly thereafter, Malgus broadcast a message across the galactic HoloNet. In this message, he criticized the apparent indifference (and subsequent disappearance) of the Sith Emperor, as well as the internal strife and xenophobic tendencies of the Dark Council and the Sith Order as a whole. Consequently, he proclaimed a "new Empire," open to all warriors willing to follow him, and decreed the dissolution of the Dark Council. Malgus' declaration caught both the Republic and the Empire by surprise, prompting both to convene emergency war councils to address the threat. With such a vast supply 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.
Taking the initiative, Malgus dispatched Serevin to lead an invasion of Ilum with the aim of destroying the Republic and Imperial forces present. 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, led 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 defeated, and the combined forces seized 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 under Malgus' command on the space station comprised a diverse array 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.
Years of combat on the front lines had made Malgus a formidable opponent, but his attackers ultimately gained the advantage. Capitalizing on a moment of weakness in Malgus' resolve, the attackers employed 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.
Following Malgus' defeat, Ilum was secured by the Republic, and the Empire successfully recovered the Adegan crystals it had extracted from the planet. However, Malgus' rebellion dealt a significant blow to the Imperial war effort; coupled with the internal conflicts among the Sith and the setbacks 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.