Following, or during, the Blackwater War, the Ninth Legion was stationed within the Murkmire region of Black Marsh. They were said to have been ruthless, inflicting death and destruction on their path, until disappearing, earning them the name the Lost Legion. The Legion was tricked by an Argonian named Gin-Rajul, who worked for the Legion as a scout during their occupation of Murkmire. Gin-Rajul brough the Legion to Tsofeer Cavern, convincing them the location would protect the Legion from the elements and threats within the area. He also convinced the Legion that a powerful artifact was located within the cave. Once the Legion had set up camp within the cave, scavenger parties and guard units started disappearing. While the Legion itself was suspicious, Gin-Rajul ensured that the troops probably deserted and fled the area. Though the Legion considered the idea insane, they did not see a reason for Gin-Rajul to lie, and so, remained within the cave.
Eventually, the Legion ended up getting attacked by the Wuju-Ka, large, strong, and resilient Voriplasm. It ended up killing many Legionnaires, resulting in what remained of the Legion causing a rockslide to keep the monster in, and prevent it from threatening the Empire's interests within Murkmire. The last Legionnaire of the Ninth Legion managed to survive for longer than most of the Legion because of the Legionnaire's skill with spellcasting. The Legionnaire thought that their trust in Gin-Rajul might have been misplaced, and that Gin-Rajul led them to the cave specifically to get killed by the Wuju-Ka. The Legionnaire would later die during his attempt to bind the Wuju-Ka, as he was attacked by voriplasms.
It was later speculated by one of Gin-Rajul's relatives that Gin-Rajul did not aid the Legion out of personal gain, but for the good of all Murkmire. The local Argonian tribes were too weak to face the Legion on their own, and so Gin-Rajul led them to the Wuju-Ka to be devoured, saving the Argonians of the area from the Ninth Legion's occupation.
During the Third Era, the Legion was selected by Emperor Uriel Septim V to aid him in his invasion of Akavir, as part of the Expeditionary Force. The Ninth Legion, alongside the Seventeenth Legion, was originally assigned as one of the reinforcement Legions after the Fifth, Seventh, Tenth, and Fourteenth had secured a landing area on Akavir.
When the beachhead was secured, however, the decision was made to send colonists over rather than the two reinforcement legions. The Ninth and Seventeenth were sent to Yneslea instead, due to local unrest, and would remain there for the rest of the invasion.
- The Elder Scrolls III: Morrowind (mentioned only)
- The Elder Scrolls IV: Oblivion (mentioned only)
- The Elder Scrolls V: Skyrim (mentioned only)
- The Elder Scrolls Online: Murkmire (mentioned only)