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)