On Kalakar Six, a moon, in 0 BBY, a duel occurred between two Sith Lords: Darth Vader and Darth Maul, who had been brought back to life via Sith alchemy. This battle was engineered by the Prophets of the Dark Side, a darksider cult, because they believed Vader was unworthy of being a Sith, "tainted" by his Jedi history, and wanted Maul to replace him as the sole apprentice of Darth Sidious.
Following the Rebel Alliance's acquisition of the Death Star schematics, Darth Sidious ordered his Sith Lord apprentice, Darth Vader, to recover the stolen data. Vader learned of a potential Rebel courier heading towards Kalakar Six, a volcanic moon that was once under Sith control. However, upon his arrival, Vader discovered no Rebel presence; instead, his stormtrooper escort was swiftly eliminated by an enigmatic assassin wielding a red lightsaber.
Vader soon found himself inside a hollowed-out volcano, where he was confronted by a darksider cult identifying themselves as the Prophets of the Dark Side. They admitted to having lured him to the moon, asserting that his Jedi past had "tainted" him, rendering him unworthy of being Darth Sidious' apprentice. The cultists then revealed that they had resurrected Darth Maul, Sidious' first apprentice, as Vader's replacement.
The two Sith Lords then engaged in a lightsaber duel, battling through the volcano and onto a bridge spanning a lava river. The bridge collapsed, but both combatants survived and continued their fight.
Vader eventually managed to bisect Maul's double-bladed lightsaber, though both halves remained functional. Maul then slashed at Vader's mask, causing him to fall. As Maul prepared the final blow, Vader used the Force to seize his lightsaber and impaled both himself and Maul through their abdomens. Vader survived due to his cybernetics. With his final words, Maul asked Vader what he hated enough to destroy him, to which Vader replied, "Myself."
Shortly after, Vader returned to confront the Prophets, only to witness Darth Sidious using Force lightning to kill them suddenly. Sidious seemed unsurprised by Vader's encounter with Maul, claiming he had arrived to assess Vader's progress in locating the stolen Death Star plans, and denying any involvement with the Prophets. When Vader inquired about Maul's resurrection, Sidious simply stated that it was possible, but inconsequential, as Maul had been resurrected only to be destroyed again. He then dismissed the cultists as heretics and instructed Vader to resume the search for the stolen plans.
Star Wars Insider 83 included the duel as one of the 20 Most Memorable Moments of the Expanded Universe, and Leland Chee confirmed its canonicity on his blog.