During the time of the Galactic Civil War, as told in Legends, the Imperial Navy put Dathomir under blockade, an effort to keep the powerful Witches of Dathomir from leaving.
Around the time that the Battle of Yavin occurred, the Galactic Empire began to establish a presence on the isolated world of Dathomir. Even though the Emperor, Palpatine, knew that Force-sensitive Witches of Dathomir were present, he still allowed a prison to be built there. Before long, however, the Nightsisters, an evil sect of Dathomiri Witches, used the dark side of the Force to corrupt the Imperial troops and turn them against their own commanders.
When the Emperor realized that the Nightsisters and their leader Gethzerion were stronger than he had thought, he gave the order to confine the Witches to their planet. Not long after the Battle of Yavin, the Emperor tasked Captain Zsinj with preventing the Nightsisters from using the Imperial starships at the prison to escape Dathomir. In response, Zsinj bombarded the Imperial prison from orbit, destroying all ships and communications equipment on the planet. All Imperial forces on the surface were abandoned, and they eventually became slaves to the Nightsisters. Zsinj then imposed a blockade on Dathomir, and the Emperor ordered that at least two Star Destroyers should always guard the planet. Imperial ships were ordered to conduct regular orbital scans to make sure the Nightsisters were still confined, with strict instructions never to land on the planet. Furthermore, orbital nightcloaks were deployed in orbit. In the years that followed, the Dathomir system remained under Zsinj's control, as he was promoted to Grand Moff of the Quelii Oversector.
Despite the Imperial blockade, the Dathomir system drew considerable attention during the height of the Galactic Civil War, becoming the site of numerous clashes between Imperial and Rebel forces. During this time, Tyber Zann, the head of the Zann Consortium, sent a defiler to disable an orbital sensor array so he could gain access to Dathomir. Along with his lieutenant Urai Fen, he landed on the planet, where he recruited Silri and a group of Nightsisters to work for him. By 1 ABY, the blockade had become quite weak, focused in only one quadrant of Dathomir space. Daring spacers often managed to trick the Imperial Dathomir Station and slip through the blockade to land at one of the two outposts on Dathomir.
After the Emperor's death at the Battle of Endor, Zsinj became a warlord and established his own empire from the Quelii Oversector. As Zsinj fought a war against the New Republic, he was no longer willing to waste warships guarding Dathomir. However, when large deposits of minerals were discovered on Dathomir's moon, Koratas, Zsinj ordered the construction of huge shipbuilding facilities in orbit around Dathomir, known as Rancor Base. Because a number of Star Destroyers were assigned to defend the shipyard, they were also tasked with maintaining the blockade of the planet.
In 8 ABY, a large Hapan fleet attacked Dathomir, destroying Rancor Base, most of Zsinj's fleet, and the orbital nightcloaks. Warlord Zsinj was killed during the battle when his flagship, the Executor-class Star Dreadnought Iron Fist, was destroyed. The blockade was then broken, as all remaining Imperial forces quickly abandoned the system, and the united Witch clans requested to join the New Republic.