The Pythia is a name as much as a title of the matriarchal leaders of the planet Gallifrey prior to Rassilon. They possessed powerful psychic powers and the power of precognition. The Pythia ruled a Gallifrey which was governed by mystery and superstition.
Near the end of her reign, the last Pythia was unable to see into the future. To see further, she recruited Vael, a Gallifreyan skilled in psychic powers, to work for her and spy upon Rassilon. When her plans failed, she sent her followers to the planet Karn, cursed Gallifrey and threw herself into an abyss below her temple. Pythia's followers would adopt the title of the Sisterhood of Karn.
The Sisterhood of Karn and Lady Peinforte possessed the remnants of the Pythia's power.