The Mother Superior was the leader of the convent of the Little Sisters of Excelis on Artaris. Though appearing human, she and Sister Jolene were in fact the last survivors of an insectoid race.
She and Grayvorn fought for possession of the Relic. They fell off a tower into a valley, with the Fifth Doctor thinking they were dead. However, Grayvorn survived and the Mother Superior's consciousness was merged with his. (AUDIO: Excelis Dawns)
Grayvorn tried for centuries to separate his mind from hers. When he was about to succeed, the Sixth Doctor stopped him in a museum. The Doctor thought the two were destroyed again, but the Mother Superior's mind went into the Relic and Grayvorn's mind merged with the museum. (AUDIO: Excelis Rising)
The Seventh Doctor returned to Artaris three hundred years later to find Grayvorn reconstituted. He was able to communicate with the Mother Superior's mind in the Relic and learn how to stop Gravyorn. (AUDIO: Excelis Decays)
When Bernice Summerfield and Iris Wildthyme visited Artaris in the early 27th century, they stopped Jolene from using the Relic and caused it to be deactivated. This freed all the souls trapped in it, including the Mother Superior's. (AUDIO: The Plague Herds of Excelis)