In the year 1000 BBY, according to Legends continuity, a fierce lightsaber battle occurred on the world of Lehon. This confrontation pitted two Sith Lords against each other: Kas'im, a Twi'lek Sith Blademaster, and his former student, Darth Bane. Bane was tracked down by Kas'im to Lehon, where he discovered him within the Rakatan Temple of the Ancients. There, he presented the Human Bane with a choice: either rejoin the Brotherhood of Darkness, the organization Bane had previously abandoned, or meet his end on Lehon. Bane's refusal ignited a brutal lightsaber duel.
Bane, possessing superior strength in the Force, initially seized the offensive. However, the battle's momentum shifted when Kas'im separated his double-bladed lightsaber into two distinct sabers and attacked with an unexpected level of intensity. Bane was forced to defend, narrowly escaping the temple before Kas'im unleashed a wave of Force energy, hurling him. Bane retaliated with his own Force attack, which Kas'im mostly absorbed by erecting a Force barrier. However, the temple entrance where Kas'im stood was shattered by Bane's power, resulting in the Twi'lek's demise as he was buried under tons of collapsing debris.
Around 1000 BBY in Legends lore, Darth Bane, who had recently ascended to the rank of Dark Lord of the Sith, deserted the Brotherhood of Darkness amidst their ongoing conflict with the Jedi Order and the Galactic Republic. Bane believed that the Brotherhood was a fundamentally flawed group whose collective embrace of the dark side had diminished its power. Consequently, he sought his own path to Sith enlightenment. This quest led him to Lehon, also known as the "Unknown World," the planet where the Jedi Civil War had concluded nearly three millennia earlier. Upon arriving, he located an ancient Rakatan temple, where he discovered and began studying the Sith holocron of Darth Revan.
Meanwhile, the remaining members of the Brotherhood of Darkness had gathered on Ruusan for a final battle against the Jedi's Army of Light. Githany, Bane's former instructor, experienced a Force-induced vision foretelling Bane's arrival on Ruusan to destroy the Brotherhood. She informed the Sith leader, Kaan, who then conspired with her to bring Bane back into the fold, hoping to avert the vision's fulfillment. Githany knew through her connection in the Force with Bane that he had gone to Lehon. Therefore, Kaan tasked Kas'im, the Twi'lek Blademaster, with finding Bane and speaking to his former apprentice on Kaan's behalf. Kaan's instructions were clear: if Bane refused to rejoin, Kas'im was to eliminate him. The Twi'lek departed in a Sith Buzzard starfighter and headed for the Unknown World.
Kas'im tracked Darth Bane by following the signal from Bane's stolen spaceship, the Valcyn, landing his starfighter nearby. He also sensed Bane's proximity and followed it to the Temple of the Ancients, fighting and killing a rancor along the way. Upon arriving in the Temple's innermost chamber, Kas'im found Darth Bane, startling him and causing him to ignite his lightsaber. They debated the Brotherhood's ability to defeat the Jedi, and the Twi'lek activated his double-bladed lightsaber as he tried to persuade Bane to join the war on Ruusan. Bane remained resolute, and when he rejected Kas'im's final offer to either rejoin or die on Lehon, the Blademaster engaged Bane in lightsaber combat.
Darth Bane quickly realized that his former master had concealed his true skills during their training. He was barely able to defend himself, which surprised Kas'im, who had expected a swift victory. Both acknowledged each other's improved abilities since their last encounter, and they adjusted their tactics before resuming the fight. Kas'im launched a relentless offensive, but Bane fully embraced the dark side and erected an impenetrable defense with his lightsaber. He then took the offensive, pushing Kas'im back and forcing him to retreat from the chamber. Bane lunged forward, narrowly missing a blow that would have crippled his opponent. Kas'im switched between various strategies and fighting styles, but Bane anticipated each change and maintained his advantage. He had seen and developed counters for every technique the Twi'lek could use with his saberstaff. This, combined with Bane's superior knowledge of the Temple's layout, allowed him to gain the upper hand.
Facing defeat, Kas'im performed acrobatic maneuvers to evade Bane's assault. Realizing that his opponent was forcing him into a dead-end hallway, Kas'im used the Force to forcefully open the door to an adjacent room, diving inside. Seeing that the Twi'lek had trapped himself, Bane stood in the doorway, savoring his apparent victory. Kas'im, though exhausted, chided Darth Bane for not seizing the opportunity to finish him. The Blademaster then twisted the hilt of his double-bladed weapon, separating it into two individual lightsabers, and attacked his adversary.
Bane was completely unprepared for Kas'im's dual-saber variation of Ataru and was quickly overwhelmed. Despite his superior Force abilities, Bane's seemingly certain victory transformed into a desperate fight for survival. Kas'im attacked relentlessly from all directions, forcing Bane to retreat in a battle he believed he could not win. His only option was to reach the Temple's entrance and escape. Kas'im realized Bane's plan as the opening came into view, but he was momentarily thrown off balance by a Force push from Bane as he back-flipped through the entrance onto the landing outside. Bane's flip carried him too far, causing him to land unsteadily on the edge of the staircase. Kas'im followed with a telekinetic blast, knocking Bane down the stairs, but Bane protected himself with the Force, avoiding serious injury.
Kas'im stood under the Temple's archway, looking down at Darth Bane as he recovered. He told Bane that escape was futile because Kas'im would pursue him until he could kill him. He suggested that Bane end the duel then, but Bane, who had been gathering Force energy, unleashed it in a devastating wave against his foe. Kas'im shielded himself with the Force, but he underestimated the destructive impact Bane's attack would have on the Temple's structure. The sheer force of Bane's energies destroyed the Temple entrance, causing it to collapse and crush Kas'im. Unconcerned with his former mentor's death, Bane left the Twi'lek under the rubble and departed for Ruusan.
News of Kas'im's death reached Lord Kaan on Ruusan before Darth Bane's arrival, delivered via a message drone sent by Bane. The message also included instructions for creating the thought bomb, an ancient Sith weapon that unleashed the pure destructive energy of the dark side. Bane hoped that Kaan, believing it to be a peace offering, would detonate the thought bomb, obliterating every Force-sensitive being on Ruusan. Upon arriving, Bane convinced Kaan to use the thought bomb during the final battle for control of Ruusan. As Bane had predicted, the thought bomb destroyed Kaan and the entire Brotherhood of Darkness, as well as most of the Jedi on the planet. Bane had indeed destroyed the Brotherhood, fulfilling Githany's vision. The elimination of his rivals allowed Darth Bane to establish a new Sith Order based on his own principles—one that concentrated the power of the dark side in two individuals: a Sith Master to embody that power and a single Sith apprentice to crave it.
The duel on Lehon is depicted in Drew Karpyshyn's first novel in the Darth Bane Trilogy, titled Path of Destruction. The event is also mentioned in The Complete Star Wars Encyclopedia, which notes Kas'im's death at the hands of Darth Bane in his entry.