The High Priestess was in charge of an ancient sisterhood called the Sibylline Sisterhood who were hired by the city of Pompeii to see the future.
Earlier she had been possessed by the Pyrovile force by breathing in volcanic dust. Her body had largely turned into stone. She and the rest of the Sisterhood had a telepathic link to each other and to several other citizens of Pompeii, such as Lucius Petrus Dextrus and a young woman called Evelina. In 79, Donna Noble told Evelina of the events that were due to come and referred to them as a new prophecy. She foretold that Mount Vesuvius would erupt and destroy the city.
The High Priestess had a plan to convert humans into Pyrovile to keep them from extinction and then boil the oceans away, taking over Earth and all civilisation. She died as a result of Mount Vesuvius erupting and destroying her along with Pompeii. (TV: The Fires of Pompeii)
