The Sisterhood of Karn is a fictional religious cult that appears in the British science fiction television series Doctor Who. Residing on the planet Karn, the Sisterhood was introduced in the 1976 serial The Brain of Morbius, and later appearing in the 2013 mini episode "The Night of the Doctor" and the 2015 episodes "The Magician's Apprentice" and "Hell Bent". They are the guardians and worshippers of the Sacred Flame, which provides for them the Elixir of Life and thus grants them immortality. They are in some way related to, and affiliated with, the Time Lords, the species of which the show's protagonist, the Doctor, is a member. Backstory In The Brain of Morbius, details of Sisterhood's religious practices are abundant and much is made of their tempestuous relationships with other species. They are led by Maren, already old at the Elixir's discovery, and worship the "Sacred Flame", which furnishes them with the "Elixir of Life" and provides them with their mighty power, thus it fades when they are away from the flame. Only the Time Lords, with whom they have shared the Elixir since the "Time of the Stones", parallel them in mental ability; while other races they can "destroy from within", the Time Lords can resist their influence – a strength which allows Morbius to reside on Karn undetected. The Time Lords themselves use their Elixir sparingly, only, for example, if they encounter difficulty when regenerating. The flame itself is, according to the Doctor, the product of gases rising from the molten core of the planet up to the surface through a geological fault, and thus is to burn for millions of years. Apparently, the Elixir tastes of "stewed apricots – no custard". They are revealed to have become paranoid after their betrayal by Morbius, who, having promised immortality to his followers, laid waste to Karn in search of their life-giving Elixir. It is further revealed that prior to Morbius' revelation, only the High Council of the Time Lords knew of the Elixir and that, since their betrayal, the Sisters have been using their considerable mental powers to crash passing spacecraft onto the surface of Karn for fear of their taking the Sacred Flame. Over a year prior to the events of The Brain of Morbius, the Flame, clogged with soot, began to fade and Elixir stopped being produced and so, by the time of the main events