Cassandra Trelawney was a celebrated seer. She was the great-great-grandmother to Hogwarts School of Witchcraft and Wizardry Divination Professor Sybill Trelawney. It is unknown whether she was part of the Hogwarts staff, for this is not stated.
Cassandra was a princess of Troy. Her beauty caused Apollo to grant her the gift of prophecy. However, when she did not return his love, Apollo placed a curse on her so that no one would ever believe her predictions.