The Sisterhood of Karn was a female society dedicated to protecting the Sacred Flame, which produced the Elixir of Life.
The Sisterhood was a remnant of the Pythia's power. (NA: Cat's Cradle: Time's Crucible)
The Sisterhood entered into the Pact of Rassilon for protection under Gallifrey they also fought against Morbius when he attempted to invade Karn. (PDA: Warmonger)
When the Sacred Flame appeared to be dying, the Sisterhood suspected the Doctor of trying to steal the remaining Elixir, but later helped him defeat Morbius. (DW: The Brain of Morbius)
After Romana became President of Gallifrey, she opened diplomatic relations with the Sisterhood. (NA: Lungbarrow)
They later tried to prevent Morbius' resurrection. BFA: Sisters of the Flame/The Vengeance of Morbius