509th Pythia
The 509th Pythia was the intended successor to the 508th Pythia.
Having read in the Book of Future Legends that the next ruler would be male, the 508th Pythia chose Vael to be her successor. However, Vael supported the Neo-Technologists and ran from this duty by joining the crew of the Time Scaphe. In an attempt to find him, the Pythia used the eye of the Sphinx to pass through the Gate of the Future and look through his mind, where she had a conversation with the Seventh Doctor. After the Doctor informed her that she was the last Pythia, she sought to control Vael into stealing his Ship. However, Vael rebelled and severed her psychic link with him, and after weeping for her loss, she cursed Gallifrey and committed suicide.
Noting that in her last words the Pythia had referred to her successor as "he", Lord Dowtroyal declared Rassilon the new ruler of Gallifrey. (PROSE: Cat's Cradle: Time's Crucible)