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.
History
Cassandra and the Great Mother met up with 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. (BFA: Zagreus)
The actions at this event were witnessed by Charlotte Pollard as part of an interactive projection by the Doctor's TARDIS, built from historical facts, but with Charley inserted into the events. It is unknown how much the projection stayed accurate to real events. (BFA: Zagreus)