The Brain Of Morbius
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Maren of the Sisterhood of Karn was the high priestess of the Temple of the Flame on the planet Karn.
Maren, like the other priestesses of the Sisterhood, guarded the Sacred Flame and the Elixir of Life the flame produced, which could grant immortality. Maren generally distrusted the Time Lords.
During the war against the renegade Time Lord Morbius, as “Reverend Mother”, Maren advised the Hospice of Karn and had authority over who might receive a sip from the elixir to heal them. Later she gathered the Sisterhood to assist the Fifth Doctor’s vast multi-species army, the Alliance, to pool together the priestess’s psychic powers to defeat Morbius. (Warmonger) After Morbius’ trial, Maren watched his execution, little guessing at the time that his brain survived the destruction of his body. (The Brain Of Morbius)
Decades later, the Fourth Doctor encountered Maren for the first time, though, in Maren’s personal timeline the two had already met. (The Doctor had used an alias at that time.) (Warmonger) The Sacred Flame appeared to be dying. She suspected the Doctor of attempting to steal the remaining elixir. However, the Doctor later “fixed” the flame by dropping a firecracker down its shaft, showing that the problem was simply a build-up of soot which was choking the oxygen from the flame.
Despite the Doctor’s insistence, Maren refused to believe that Morbius had returned. After Kalia was killed, she allowed Ohica, her de facto deputy, to lead a party against the aggressor whom they discovered to indeed be their old enemy.
Finding the Doctor near death, Maren gave up her ration of elixir to save the Doctor’s life, accepting his view that immortality came at the expense of progress. Maren appeared to her sisters in the Sacred Flame, becoming youthful before disappearing. (The Brain Of Morbius)
After Maren’s death, Orthena became the leader of the Sisterhood. (The Vengeance Of Morbius)