The Great Hyperspace War, a clash between the Galactic Republic and the Sith Empire, unfolded a little over 5,000 years prior to the Imperial Era. This conflict was ignited by the advent of the hyperdrive and faster-than-light travel capabilities. By the time of the Imperial Era, the now-legendary war was retold in the Qel-Droma Epics, considered quasi-historical accounts. The Great Sith Wars had their genesis around this same period.
The Qel-Droma Epics place the Great Hyperspace War around 5000 BBY. It supposedly began with the Sith Empire's invasion of the planet Empress Teta, formerly known as Koros Major, which had been recently conquered by Empress Teta. The stories claim that, after the war began, Teta joined forces with the Jedi Order to combat the Sith, successfully repelling the Sith Lord Naga Sadow and dispersing the Sith forces throughout the galaxy. The Jedi Master Ooroo participated in the Great Hyperspace War, and perishing during the fighting. The inhabitants of Empress Teta presented the events of the Qel-Droma Epics, including the Great Hyperspace War, as factual history, but archaeological expeditions were unable to corroborate these legends. The Great Library of Cinnagar, however, regarded the Qel-Droma Epics as historically accurate. Consequently, the Great Library housed an extensive museum dedicated to the era of the Unification Wars and the subsequent Great Hyperspace War, displaying artifacts and artistic representations of key figures and war machines from that time.
The De Agostini magazine Star Wars: Build the Millennium Falcon first mentioned the Great Hyperspace War in its forty-seventh issue, released around November 25, 2015. The 2014 novel Tarkin alludes to a war fought against the Sith during the Old Republic era, where the Sith managed to invade Coruscant, mirroring the Legends depiction of the conflict. However, the 2016 roleplaying sourcebook Nexus of Power references the conflict via the in-universe Qel-Droma Epics, so any connection between the canonical Great Hyperspace War and the war described in Tarkin remains unclear.