Around 3636 BBY, the Eternal Empire, an emerging power originating from the depths of Wild Space, embarked on a campaign to conquer the Core Worlds. This offensive led to the defeat of both the Galactic Republic and the Sith Empire, which at the time were the dominant forces in the known galaxy.
For a significant portion of the Sith Emperor's time away from governing his Sith Empire, he was secretly establishing a new power base. This became the Eternal Empire, hidden in the uncharted regions of Wild Space, with its central hub located on the planet of Zakuul. There, under the guise of Emperor Valkorion, he meticulously built up his military might with the ultimate goal of emerging to seize control of the galaxy. By 3636 BBY, the Eternal Empire had eliminated the few Sith Empire outposts in Wild Space to gauge its strength, though the Sith Empire remained unaware of the perpetrators. That same year, as the Galactic War intensified between the Empire, Republic, and other factions, the Eternal Empire launched a series of devastating raids, targeting planets such as Korriban, before retreating back into the obscurity of Wild Space.
Darth Marr, the de facto leader of both the Dark Council and the Imperial Military, spearheaded a joint Imperial-Republic expedition into Wild Space in pursuit of the Emperor. However, they were ambushed by the formidable Eternal Fleet. Marr, along with another significant individual who would later be known as "the Outlander," was captured from Marr's destroyed flagship by Prince Arcann, Valkorion's surviving son, and taken to Zakuul to face the Emperor. Valkorion offered to share his immense power with his captives, demanding only their allegiance. Marr, recognizing Valkorion as the Sith Emperor, vehemently refused and was executed for his defiance. When the same offer was extended to the Outlander, Valkorion was killed, and Arcann took his place as Emperor. The new Emperor publicly accused the Outlander of assassinating his father and vowed that the Core Worlds would suffer the consequences. Arcann, sensing that Valkorion's essence still lingered within the mind of his "murderer," had the Outlander frozen in carbonite.
Arcann's military campaign was characterized by its speed and brutality. From his seat on the Eternal Throne, the new Emperor simultaneously launched attacks against both the Republic and the Empire, obliterating their shipyards and imposing blockades on major trade routes. The Eternal Empire possessed both superior numbers and speed. Only ships equipped with isotope-5 stood a chance of outmaneuvering the Eternal Fleet's powerful vessels. Within a mere three months, they had established naval dominance over both factions, and by the end of the first year, the Republic's capital, Coruscant, and the Imperial capital, Dromund Kaas, were under blockade. The Dark Council of the Sith Empire continued to fight, but without Marr's leadership, it was effectively destroyed. Similarly, the Republic's Supreme Chancellor, Leontyne Saresh, refused to concede the Republic to Zakuulan control. In an act of defiance, she waged war against both the Eternal Empire and the Sith Empire, resulting in the loss of millions of Republic lives. Saresh was swiftly overruled by the Galactic Senate, which sought peace. The Minister of Logistics followed suit for the Empire.
Both sides dispatched envoys—Senator Evran for the Republic and Darth Vowrawn for the Empire—to Zakuul, where they were brought before Arcann. The Emperor agreed to a ceasefire, contingent upon the Republic and the Empire paying substantial tribute to Zakuul and accepting limitations on their military strength. With few alternatives, both parties acquiesced. In the ensuing years, Saresh, though no longer in power, continued to influence policy through her successor, Jebevel Madon. Likewise, with the Dark Council virtually defunct, its sole remaining visible member, Darth Acina, proclaimed herself Empress of the Sith. Despite this, the conflict between the Republic and the Empire persisted—Saresh remained unwilling to consider any alliance with the Sith to overthrow the Eternal Empire—and Arcann permitted it to continue, provided they acknowledged Zakuul's supremacy and maintained their tribute payments.
Instead of relying on occupation forces to govern his newly acquired territories, Arcann deployed numerous massive battle stations known as Star Fortresses in orbit around key planets, including Alderaan, Belsavis, and Voss. These stations were commanded by Exarchs—Knights of Zakuul who had undergone biological and cybernetic enhancements—and were powered by large "sun generators," essentially artificial stars (similar to those used on Zakuul itself). Shield generators on the planet's surface provided protection. Any attempts at resistance were met with the stations releasing excess plasma from the sun generators in the form of devastating beams aimed at the surface, effectively holding the planets and their populations hostage.