In the novelDune, the Baron Vladimir Harkonnen has this impression of Mohiam: In Dune Messiah, Herbert notes that Mohiam's eyes are "dark with the blue brilliance of her melange addiction." Novelist Brian Herbert, Frank Herbert's son and biographer, describes Mohiam as "a witch mother archetype". The non-canon Dune Encyclopedia by Willis E. McNelly invents an extensive, alternate biography for Mohiam, including the first suggestion that she is Lady Jessica's mother.
As Dune begins, the "old crone" Reverend Mother Mohiam comes to Caladan to "test" young Paul Atreides before his departure for Arrakis. Having trained Paul's mother Lady Jessica at the Bene Gesserit school on Wallach IX decades before, Mohiam now holds a lethal gom jabbar to Paul's neck. If he withdraws his hand from a box that inflicts pain through direct nerve induction, the gom jabbar will kill him instantly. Mohiam explains to Paul that the purpose of the test is to discover whether Paul is "truly human", by requiring him to endure pain and still act rationally. He passes the test, and Mohiam is somewhat intrigued by the potential she sees in 15-year-old Paul, as she notes he has survived more pain than any other Bene Gesserit inductee. Later, Mohiam accompanies Emperor Shaddam IV to Arrakis in her role as Imperial Truthsayer; there she is faced with Paul's four-year-old sister, the pre-bornAlia, whom she calls an Abomination. Mohiam is further alarmed as she witnesses the ritual battle-to-the-death between Paul and Feyd-Rautha. Paul calls Feyd "cousin," revealing that Paul is aware of the secret that the Baron Harkonnen is his grandfather. But Mohiam is most concerned with the potential outcome of the duel, which could prove to be "a major catastrophe for the Bene Gesserit breeding scheme." She realizes that "Two end products of this long and costly program faced each other in a fight to the death that might easily claim both of them. If both died here that would leave only Feyd-Rautha's bastard daughter, still a baby, an unknown, an unmeasured factor, and Alia, the Abomination." Paul is victorious; seeing the inevitability of the situation he has orchestrated, Mohiam compels Shaddam to give in to Paul's demands, allowing Paul to depose him and become Emperor in his place.
In Dune Messiah, the Reverend Mother joins the conspiracy to topple the rule of Paul Atreides after he ascends the Golden Lion Throne and unleashes his Jihad upon the Empire. She, along with the TleilaxuFace DancerScytale, the Spacing Guild NavigatorEdric, and Paul's embittered consort Princess Irulan of House Corrino, plot to insinuate the Tleilaxu ghola Hayt into the court to weaken Paul's confidence in his prescience and thereby ruin him. Forbidden to ever return to Arrakis, Mohiam is taken into Paul's custody when it becomes known that she is on a heighlinerin orbit above the planet. Aware of the Bene Gesserit desire to acquire his genetic material for their breeding program, Paul seeks to negotiate with Mohiam. In exchange for the guaranteed safety of his concubine Chani, and the Sisterhood's acceptance of his decision to father no heirs with the Princess Irulan, Paul offers something of the utmost value: his sperm. This is a complicated proposition for Mohiam, however, because artificial insemination is forbidden in the wake of the Butlerian Jihad. When the conspiracy ultimately fails and Scytale is killed, Edric is executed in 10,207 A.G. by Fremen NaibStilgar, on orders from Alia. Stilgar also puts Mohiam to death, despite previous orders from Paul to spare her life.
''Prelude to Dune''
The Prelude to Dune prequel novels reveal that Mohiam is secretly the mother of Jessica and grandmother of Paul. According to the authors, this fact was pulled directly from Frank Herbert's working notes for the original Dune series. Mohiam is instructed by the Bene Gesserit to collect the genetic material of Baron Vladimir Harkonnen for their breeding program. She blackmails the homosexual Baron into fathering a child with her. When that daughter proves genetically undesirable, Mohiam kills it and then returns to Harkonnen for a second try, at which point he drugs and viciously rapes her. She exacts her retribution by infecting him with a disease that later causes his obesity. Mohiam kills her first child with Harkonnen, and their second child is Jessica, Harkonnen's paternity having been previously established in Dune. It is also revealed in the prequels that the name given for Jessica's mother in the original novels, Tanidia Nerus, is an alias used by Mohiam, a common practice among the Bene Gesserit to conceal true parental identity.
Mohiam is portrayed by Siân Phillips in David Lynch's 1984 film Dune, and by Zuzana Geislerová in the 2000 miniseries Frank Herbert's Dune and its 2003 sequel Frank Herbert's Children of Dune. The character will be played by Charlotte Rampling in the 2020 Denis Villeneuve film Dune. Variety wrote of the 1984 film that "Siân Phillips has some mesmerizing moments as a powerful witch". Phillips said that she and Lynch both independently came up with the idea for her character's distinctive bald appearance. Emmet Asher-Perrin of Tor.com noted of Children of Dune, "There are other moments of perfect execution... The dual conversations between Irulan and Reverend Mother Gaius Helen Mohiam—and later Jessica—are gorgeous, offering subtitles to their sign language while an entirely different conversation plays out in words."