The Great Mother was the head of the Sisterhood of Karn. She could read Old High Gallifreyan, which was useful to Lord Provost Tepesh when they entered Rassilon's Foundry. Even though she disliked Rassilon, she sided with him once she found out that Tepesh and Ouida were Great Vampires after they killed her fellow sister Cassandra.
While dealing with the aftermath of the explosion of anti-time, the TARDIS created a series of holographic projections, taken partly from reality and partly from the memories of the Eighth Doctor, Charley Pollard and the TARDIS itself. One such projection was the meeting in Rassilon's Foundry as the TARDIS looking for information on the Divergence. Inside this projection, it placed Charley in the role of Rassilon and itself in the role of the Great Vampire Arata. The projection of the Great Mother was created using the image of the Sixth Doctor's companion Evelyn Smythe. (AUDIO: Zagreus)