The Sisterhood of Karn was a Gallifreyan female society related to, and affiliated with, the Time Lords, dedicated to protecting the Sacred Flame, which produced the Elixir of Life. They originally came from Gallifrey, but resided on the planet Karn.
Prior to the Last Great Time War
The Sisterhood was a remnant of the Pythia's power. The Sisterhood were sent by the last Pythia from Gallifrey to the planet Karn after the Great Schism. (PROSE: Cat's Cradle: Time's Crucible) They had been banished from Gallifrey by Rassilon. (AUDIO: Light the Flame)
The Great Mother of the Sisterhood and her High Priestess Cassandra, from the House of Jade Dreamers, plotted to destroy Rassilon's Foundry along with Lord Provost Tepesh and Lady Ouida from "the Committee of Three". Tepesh and Ouida, who were also Great Vampires, murdered Cassandra before all were killed by the automated sterilisation of the Foundry. (AUDIO: Zagreus)
Iris Wildthyme was once one of the Sisterhood, (PROSE: Verdigris) a group she likened to Pan's People from Top of the Pops; she joined the group after her bus was brought down onto a "ghastly planet" were she was found by a welsh surgeon who lived with his hunchbacked boyfriend, though she soon escaped them after, being rescued by the group of girls who lived in caves. (PROSE: From Wildthyme with Love) Being a member of the Sisterhood was a fact she rarely shared with people, not even the Doctor, who knew not of her participation with the group when they chanted at him. As she kept her identity a secret during her time in the Sisterhood, when Ohica found out she used a Time Scoop to take many of Iris' incarnations to the Death Zone. (PROSE: Verdigris)
When the Sacred Flame appeared to be dying, the Sisterhood suspected the Fourth Doctor of trying to steal the remaining Elixir, but later helped him defeat Morbius. In reality, the only thing wrong with the Flame was that centuries of ash and soot were finally beginning to clog the underground fissure that fed the flame natural gas. The Doctor easily solved the problem by dropping a simple firecracker down the shaft. (TV: The Brain of Morbius)
Skagra learned the location of The Worshipful and Ancient Law of Gallifrey by capturing and torturing a member of the Sisterhood. He drugged her with synaptrol, an anti-telepath drug derived from the Lethe flower, and kidnapped her; he then withheld food and water from her until she told him that the book was with Professor Chronotis in Cambridge. (PROSE: Shada)
The Sisterhood were capable of teleportation using the Sacred Flame. They later tried to prevent Morbius' resurrection; failing to do this they saved the Eighth Doctor after he threw himself and Morbius into the core of Karn. (AUDIO: Sisters of the Flame / The Vengeance of Morbius)
During the Last Great Time War
When the Eighth Doctor crashed on Karn and died, the Sisterhood recovered his body and used their Elixir to temporarily resurrect him. They were unable to bring him back permanently due to the severity of his injuries but instead offered to use their Elixir to control his regeneration so he could choose the form and personality he took. The Sisterhood explained that they were helping him due to owing him for the last time and because they believed that he was the only hope of stopping the Last Great Time War without destroying the universe. In the end, the Doctor agreed and with the Sisterhood's help, regenerated into his next incarnation. (TV: The Night of the Doctor) Unknown to the Doctor, the so-called Elixir that he thought controlled the regeneration was only lemonade and dry ice. (PROSE: The Day of the Doctor) Afterwards the Sisterhood gave Cass a funeral.
The Doctor stayed with the Sisterhood as he recovered from his regeneration, being guided through the Time War wreckage on Karn by sister Lithea. Ohila allowed two Time Lords, Cardinal Rasmus and Commander Sanmar, to come to Karn, unaware Sanmar had secret orders to extract the heart of Karn. After Sanmar killed Lithea, all the Sisterhood felt it and rallied the forces of Karn against her. The Doctor secured a pledge from Commodore Tamasan to protect Karn from an oncoming war front with a time lock, however just before it was in place the Sisterhood used their powers to make Karn disappear anyway. (AUDIO: Light the Flame)
After the Last Great Time War
After meeting (and abandoning) a young Davros on the battlefields of Skaro, the Twelfth Doctor visited Ohila on Karn, hiding himself from the universe so effectively that no one could find him. While the Sisterhood gave him refuge, the servant of Davros known as the Colony Sarff visited Karn in their search for the Doctor. However, Ohila warned Sarff that the Doctor was not one to be trifled with, and when the Colony became angered and threatened the Sisterhood she informed them that their powers were useless on Karn. Thwarted, Colony Sarff departed, leaving Ohila with an ominous message for the Doctor — "Davros knows, Davros remembers." (TV: The Magician's Apprentice)
Seeing that the Doctor was deeply troubled by this, Ohila attempted to comfort him, and warned him against heeding the summons of his old foe, but the Doctor could not be dissuaded. After failing to change his mind about the meeting, Ohila agreed to deliver the Doctor's confession dial. The Doctor told her that he would stay a little while before going to meet his ancient enemy. (WC: Prologue) The Doctor's confession dial was then delivered to Missy. (TV: The Magician's Apprentice)
Some members of the Sisterhood visited Gallifrey on hearing the Twelfth Doctor had returned from the torture of his confession dial, Ohila citing the reason for her visit as "One so loves fireworks." (TV: Hell Bent)