The Ninth Legion was a Roman legion that was active in the 1st and 2nd centuries. Its ranks ranged from Legionary (First Class) to Legionary (Fourth Class). In 64, the Legionary (Second Class) Ascaris was removed from regular duties and given the special task of assassinating Maximus Pettulian, a lyre player who played to kill Emperor Nero. After he seemingly failed in his task, due to the First Doctor assuming Maximus Pettulian's identity, Ascaris was demoted to Legionary (Fourth Class). (PROSE: The Romans)
In the 2nd century, the Ninth Legion invaded Aberdeen, Scotland. The Gatekeeper at the time, Kar, allowed a single Eater of Light to pass through the portal; she thought it would destroy the legion, and become so weakened in the process that it would be easy to defeat. Instead, the creature became stronger after feeding on the Romans and began to darken the sky, threatening the world.
The Twelfth Doctor, along with his companions Bill Potts and Nardole, worked with the Picts and the few surviving Romans to drive the Eater of Light back into the portal. Kar, along with several Picts and the Romans, then entered the portal to fight off the creatures. (TV: The Eaters of Light)
With Castle Extremis in mind, the Tenth Doctor mused that the soldiers of the Ninth Legion "could have slept safely on their bunks if only Hadrian had opened his wall to tourists". (PROSE: Martha in the Mirror)