Mother Superior is the title reserved for the most senior Reverend Mother in the Bene Gesserit Sisterhood. Mother Superiors were essentially supreme commanders of the Sisterhood, and held final authority on all decisions.
Raquella Berto-Anirul is the first to ever undergo the Agony, when the jealous Sorceress Ticia Cenva poisons her. Instead of dying, Raquella's body adapts the poison, gaining access to Other Memory, turning her into the first Reverend Mother. After Ticia's death, Raquella forms the Sisterhood out of the remains of the Sorceresses of Rossak and becomes the first Mother Superior.
After Emperor Salvador Corrino disbands the Sisterhood, leaving only a small Orthodox faction on Salusa Secundus headed by the anti-technological Reverend Mother Dorotea (Raquella's own granddaughter), Raquella moves the Sisterhood to the inhospitable Wallach IX. Despite considering herself to be the leader of the one true Sisterhood, Dorotea never formally titles herself Mother Superior. Nearing her death, Raquella summons Dorotea and her bitter rival Valya Harkonnen (also a Reverend Mother). Raquella manages to convince the two Reverend Mothers to reconcile their differences and work for the betterment of the Sisterhood. On her deathbed, Raquella names both Reverend Mothers as her successors, urging them to lead as partners. However, immediately after Raquella dies, the vengeful Valya uses the Voice to have Dorotea commit suicide, leaving her as the sole Mother Superior.