The Mother Superior was a vessel of Abaddon.
In 1958, Abaddon takes possession of the Mother Superior of the St. Bonaventure Convent in Milton, Illinois. Abaddon and her demons were planning on stealing souls. After one of the soulless nuns, Mary Catherine, kills two people and commits suicide, the convent is visited by Henry Winchester and Josie Sands, who claim to be from Office of the Inquisition.
Abaddon, under her guise, permits the two to search Sister Mary's room. She sends Sister Julia Wilkinson to act as their guide.
That night, Julia witnesses Abaddon dragging a struggling young woman across the floor. She sees Abaddon's eyes turn black. Sister Agnes, who is possessed, knocks Julia out and takes her to the room where they're keeping kidnapped humans. Julia is tied to a chair with a few other people. Sister Agnes begins dragging each people away to steal their soul, until only Julia is left.
Before the possessed nuns can harm Julia, Henry and Josie arrive and free her, as well as exorcise to the nuns. When the Mother Superior appears, the exorcism fails to work and Abaddon merely flings Henry into a wall, knocking him out. Abaddon then berates the two "hunters" for getting involved in other people's businesses. When Josie reveals she is a member of the Men of Letters, Abaddon is intrigued.
When Abaddon reveals her intention to use Henry as a vessel so she can study the Men of Letters up close, Josie insists that Abaddon take her instead. Although she revels in the fact that Josie has unrequited feelings for Henry, she agrees to take Josie and proceeds to abdicate the Mother Superior.
The Mother Superior collapses, indicating that she was either dead or unconscious.