The clash on Vjun, known as the Battle of Vjun, involved the New Republic in military action against X1's faction during the era of the Galactic Civil War.
Following the Battle of Endor, the deaths of both Emperor Palpatine and Darth Vader seemed to signify the end of the Sith threat. This pivotal moment enabled the Rebel Alliance to begin its transformation into the New Republic. However, X1, a former clone trooper from the days of the Galactic Republic and a member of the Galactic Empire, survived the destruction of the Death Star II above Endor. With the Emperor gone, X1 aimed to establish a new Galactic Empire, placing himself as its ruler and declaring himself a Sith Lord. Despite his ambitions, X1 lacked sufficient understanding of the dark side of the Force and sought knowledge to bolster his position. Consequently, he led Imperial forces loyal to him to the planet of Vjun, intending to infiltrate Bast Castle, Darth Vader's fortress, in search of dark arts secrets.
Upon discovering that his brother had survived, his clone counterpart, X2, resolved to thwart him and journeyed to Vjun with Jedi Master Luke Skywalker. They had obtained intelligence suggesting that X1 had established his new base of operations at Darth Vader's castle, defended by his Imperial forces.
Upon entering the Vjun system, X2 focused on neutralizing the planetary security satellites to prevent Imperial forces from detecting enemy presence. Following instructions from the Jedi Luke Skywalker, X2 infiltrated the orbiting Imperial Star Destroyer, which was unaware of the Republic's presence. Once aboard, the former clone trooper engaged stormtroopers in combat to access the ship's database and gather information about the forces stationed on Vjun's surface. After slicing into the computer systems, he acquired the necessary data, left the capital ship, and proceeded to the planet's surface. Skywalker later informed him that the intelligence obtained from the Imperial warship indicated that X1 had already departed the planet but also revealed the presence of a Sith holocron within Bast Castle, requiring retrieval.
However, the Imperial forces were now fully alert and deployed TIE/LN starfighters to defend the installation. Additionally, stormtroopers were both garrisoned and mobilized to confront any intruders. This forced X2 to use his X-wing to eliminate the enemy fighters before landing at the hangar, where he engaged in combat with the defending stormtroopers. While battling Imperial forces, X2 received information from Skywalker that the data from the Star Destroyer showed X1 had taken his genetic father Falon Grey's lightsaber and placed it in Darth Vader's trophy room. Fighting through waves of Imperial soldiers, X2 successfully entered the trophy room and recovered his genetic father's lightsaber, which had been lost on Dantooine.
Subsequently, he sought to locate and retrieve the Sith holocron. As he entered the inner sanctum, X2 was confronted by security droids left behind by Darth Vader. After defeating his attackers, X2 discovered the holocron within the meditation chamber located deep inside Bast Castle. Upon securing the holocron, he was attacked by more Imperial troopers and security droids, compelling him to fight his way back to the hangar to escape and return to Republic forces. The retrieval of the holocron enabled Luke Skywalker and Republic engineers to decode it, revealing X1's objectives and his next destination.