Pythia

The Pythia was a name as much as a title of the matriarchal leaders of the planet Gallifrey prior to Rassilon, who possessed powerful psychic powers, and had the power of precognition. The Pythia ruled a Gallifrey which was governed by mystery and superstition. The 389th Pythia gave the legendary hero Ao a jeweled periapt with a tongue of blue ice.

Near the end of her reign as Pythia, the last Pythia was unable to see into the future, in an attempt to see further she recruited Vael, a skilled Gallifreyan with psychic powers, to work with her and then spy upon Rassilon. When her plansof her future failed to come she sent her followers to the planet Karn and then cursed Gallifrey with sterility and threw herself into an abyss below her temple. Pythia's followers would eventually 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 got lifted, so that the Human Leela and Andred could conceive a child together. (NA: Lungbarrow)