Cassandra was a Time Lady visionary who became a vampire in an alternative timeline visited by Romana II. (AUDIO: Annihilation)
Alternative timelines
In Romana's original timeline, there had also been a Cassandra — a Prophetess and assistant to the Great Mother of the Sisterhood of Karn — during the foundation Time Lord society. In this timeline, Cassandra was killed by the vampire Time Lords Tepesh and Ouida. While dealing with the aftermath of the explosion of Anti-Time, the Eighth Doctor's TARDIS created a series of holographic projections, including the death of Cassandra, for whom it used the image of the Doctor's companion Melanie Bush. (AUDIO: Zagreus)