Cassandra (The Myth Makers)

Cassandra was the High Priestess of Troy and the daughter of King Priam. The First Doctor's companion Katarina served as her handmaiden. She was the sister of Paris, Hector and Troilus.

Biography
For an unknown time prior to the arrival of the Doctor's TARDIS, Cassandra had had nightmares of the future. A mean-spirited and vindictive sort, she nevertheless warned Priam and those associated with her that doom would come to Troy, the people of which had long failed to take her seriously. When Vicki (later Cressida) emerged from the TARDIS, which the Trojans had taken to the city, she immediately accused her of practising witchcraft. She warned of the Trojan Horse (a plan put into action by the First Doctor, borrowing the idea from Homer), but to no avail.

After the fall of Troy, she became the slave of the Greek warlord Agamemnon. (DW: The Myth Makers)