One of the major antagonists of Star Wars: Knights of the Old Republic II: The Sith Lords, Sion is probably the closest the Star Wars setting has ever come to a zombie character. After serving under both Exar Kun and Darth Revan, Sion became a Sith Lord under Revan's empire. Once Revan defeated Darth Malak and departed into the deep reaches of space, the Sith turned against each other. Darth Sion and Darth Nihilus were among the few that survived the chaos. Trained by the fallen Jedi Master Darth Traya at Malachor V, Sion's power grew and together with Nihilus, he turned on his former Master. He attempted to hunt down and kill the remaining members of the Jedi Council but was stopped by the Jedi Exile. Ironically trained by Traya as well, the Exile finally caught up to Sion in the Sith academy on Malachor V where they had both fought against the Mandalorians under Revan's command years before.
Sion's only actual appearance was in The Sith Lords, but he made a cameo appearance in Star Wars: The Force Unleashed as an optional skin to replace the main character, Starkiller. This was purely cosmetic; although the player took on the appearance of Darth Sion, the game still played exactly the same, and "Sion" still held his lightsaber behind his back, in Starkiller's ready stance.