In the year 19 BBY, according to Legends continuity, Darth Vader, the Sith Lord, and Sha Koon, a Jedi Knight, engaged in a duel. This confrontation occurred in the depths of the Coruscant Underlevels, situated below the Jedi Temple.
During 19 BBY, as depicted in Legends, Sha Koon, a Jedi Knight who managed to survive Darth Vader's assault during Operation: Knightfall, made contact with Emperor Palpatine. She attempted to deceive him by suggesting she desired to become his new Sith apprentice, offering Vader's death as proof of her loyalty. However, Koon's true goal was to assassinate the newly appointed Sith Lord. Palpatine, aware of her deception, dispatched Darth Vader to eliminate the escaped Jedi, confident in his apprentice's ability to triumph.
Vader entered the underlevels, which Koon had claimed as her territory, and was immediately set upon by Cthons. Koon was using the Force to control these creatures. The Cthons ensnared Vader in an electromagnetic net, nearly disabling the Sith's life support systems. However, the former Jedi managed to break free, violently dispatching the Cthons before turning his attention to Koon. A lightsaber battle ensued, during which Koon managed to slash Vader across the chest. Ultimately, Vader gained the advantage, using the Force to grip her throat before impaling her with his lightsaber, resulting in her death.
As she died, Koon had a vision. She saw Vader's eventual redemption and the rebuilding of the Jedi Order, and found peace in this vision. Darth Vader believed that he had potentially eliminated the last of the Jedi with Koon's demise.