Collis was a Time Lord soldier during the Last Great Time War, although she protested to the War Doctor that she wasn't one. She was sent by Cardinal Ollistra to find the Annihilator and met the War Doctor who wanted to destroy the weapon.
She died fighting the Daleks in Vildar after touching a Varga plant, telling the Doctor that she was a clerk before the war. The Technomancers brought her back to life, leading to the Doctor asking her about the process. She didn't think the Vildaran sacrifice was just to resurrect Time Lords and eventually realised that she had part of the Horned Ones in her, sacrificed herself again to rid the universe of the Horned Ones. (AUDIO: Legion of the Lost)