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.
When the Tenth Doctor demanded the Pyrovile mutating her body to reveal itself, the High Priestess effectively was murdered by it; the Pyrovile took over her body and began commanding the foolish Sisthood into following the plans of its race. As she screamed in rage at Mount Vesuvius erupting, the mutant was destroyed along with Pompeii. (TV: The Fires of Pompeii)
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