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, which later histories contrasted with the reason and science of the Rassilon Era. The 389th Pythia was given a jeweled periapt by the legendary hero Ao.
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. (PROSE: 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 his former human companion, Leela, and her Time Lord husband, Andred, could conceive a child together. (PROSE: Lungbarrow)
There were, however, some instances of womb-born children on Gallifrey between the curse and Leela's pregnancy, although Rassilon issued a decree that made them illegal. In at least one instance, it was enforced by abortion and execution. (PROSE: Cold Fusion, The Infinity Doctors)
The Sisterhood of Karn and Lady Peinforte possessed the remnants of the Pythia's power, (PROSE: Cat's Cradle: Time's Crucible) and Kadiatu Lethbridge-Stewart would dream of her, as the Pythia's curse would send massive repercussions through the metareality of the space-time continuum. (PROSE: Transit)