The incident referred to as the Disaster at Deepspace Besh took place nine months following the Battle of Yavin.
Darth Vader aimed to conclude the Galactic Civil War swiftly in a single, crucial engagement, utilizing Janek Sunber's efforts to capture Luke Skywalker alongside Jorin Sol's mission to disable the Rebel command ship.
Immediately following Jorin Sol's attempted assassination of the commanders aboard Rebel One, the Imperial fleet emerged from hyperspace and began firing on the Rebel forces, causing widespread destruction of transports and smaller capital ships.
Jorin Sol, filled with remorse after regaining his senses, successfully persuaded Rebel spymaster Tungo Li to instruct all Rebel vessels to disperse into hyperspace rather than adhere to the now-compromised escape algorithms, which would only lead them to another Imperial fleet.
Contrary to Vader's expectations, Wedge Antilles's starfighters managed to delay the Imperial fleet, giving Sol enough time to overcome his programming and set the damaged Rebel One for a hyperspace jump. Luke Skywalker then convinced Sunber to release him and Princess Leia Organa, allowing the Rebel ships to escape through hyperspace and scatter across the galaxy. A significant portion of the fleet was considered permanently lost. Despite the heavy losses suffered by the Rebel fleet, their escape represented a strategic failure for the Empire. Although Vader's scheme to crush the Rebel Alliance in one decisive blow resulted in the destruction of several major capital ships and the eventual loss of Rebel One, the Alliance Fleet was rebuilt with support from allies like the Mon Calamari and ultimately triumphed over the Imperial Navy at the Battle of Endor more than three years later in 4 ABY.
The Essential Atlas specifies that the Deepspace Besh disaster occurred in grid square P-13, located within the Sepan sector of the Expansion Region.