The clash between Luke Skywalker and the mentally unstable Jedi Knight Celeste Morne, a Human female, took place on an isolated moon orbiting a gas giant. This lightsaber duel, known as the Second duel on unidentified desert moon, occurred nine months after the Battle of Yavin.
Back in 19 BBY, the Sith Lord Darth Vader acquired Dreypa's Oubliette, an ancient Sith artifact that induced stasis on whoever was inside. He believed this oubliette held both the powerful Sith artifact called the Muur Talisman and a Jedi from a bygone era. Hoping to harness the Talisman's power and take the mysterious Jedi as his apprentice, Vader opened the oubliette, and Celeste Morne, a healthy female Jedi, emerged. Upon realizing Vader was a Sith Lord, a duel with lightsabers erupted between them. Despite Vader disarming her, Celeste employed the Muur Talisman to transform Vader's stormtroopers into rakghouls, compelling him to flee the moon, inadvertently leaving her stranded.
Roughly nineteen years later, Vader's thoughts returned to Celeste, and he considered exploiting her against the ongoing Rebellion opposing the Sith-led Empire. He dispatched an Imperial convoy to ascertain Celeste's survival. As the convoy approached the moon's surface, transmissions ceased when Celeste transformed the pilots into rakghouls, resulting in the shuttles crashing. Confirming Celeste's continued existence, Vader instructed former Rebel spy, Wyl Tarson, to transmit a message to the Rebellion about a clandestine Imperial weapon. Consequently, a Rebel team, spearheaded by Luke Skywalker and Princess Leia Organa, landed on the moon to investigate the message.
Celeste had spent nearly two decades in solitude on the moon, haunted by the spirit of the Sith Lord Karness Muur, driving her to the brink of madness and fostering a deep-seated hatred not only for Vader but also for the Jedi Order. Thus, as the Rebels began investigating the wreckage of the Imperial shuttles, the rakghouls under Celeste's control attacked them, dragging Luke and Leia into a cave where Celeste awaited, poised to strike.
Celeste Morne initiated the battle by attacking Luke Skywalker. Luke managed to block the initial blows, however, Celeste then used telekinesis to overwhelm Luke Skywalker. Forced to retreat, Luke sustained several hits from Celeste Morne before being kicked aside and defeated. Morne then held her Lightsaber next to the throat of a defeated Luke.