Cassandra was High Priestess of the Sisterhood of Karn during the time of Rassilon. She worked as Prophetess and assistant to the Great Mother of the Sisterhood. She belonged to the House of Jade Dreamers, one of the lesser of the Great Houses of the Time Lords.
Cassandra and the Great Mother met up with Lord Provost Tepesh and the Lady Ouida on the plains of Gallifrey soon after Rassilon had stabilised the Eye of Harmony. The four of them broke into Rassilon's Foundry, looking for evidence of crimes. When Cassandra warned that events were not as they seemed, Tepesh and Ouida revealed themselves as Great Vampires. At Tepesh's command, Ouida swiftly drained the blood from Cassandra, killing her.
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 was 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 Cassandra was created using the image of the Sixth Doctor's companion Melanie Bush. (AUDIO: Zagreus)