The Pythia was 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. The 389th Pythia was given a jeweled periapt by the legendary hero Ao.
History
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 with sterility and threw herself into an abyss below her temple. Pythia's followers would adopt the title of the Sisterhood of Karn. (NA: Cat's Cradle: Time's Crucible)
As a result of the curse, Gallifreyans (and Time Lords) could no longer reproduce naturally, so they created the Looms. During the Doctor's seventh incarnation, the curse was lifted, so the human Leela and Andred could conceive a child together. (NA: Lungbarrow)
The Sisterhood of Karn and Lady Peinforte possessed the remnants of the Pythia's power. (NA: Cat's Cradle: Time's Crucible)