Sashana was a psychic imposter.
Sashana used her abilities to make Matildus Galathea, the head librarian of the Renath Archive library in Cornucopia's Kaballus Quarter, think Sashana was her granddaughter. Sashana "psychically chipp[ed] away" at Matildus for months, making her doubt herself, and she blocked Matildus' memories, making her memory appear to fail. This was part of a plot to have Matildus pass on custody of the archive to Sashana so she could sell off the library's collection.
Bill Potts revealed Sashana's deception, and told the Twelfth Doctor, Matildus and Sashana to stop whatever it was they were doing. Bill took a photograph on her phone which revealed Sashana's true form. She began to fry everyone's brains with her mind, but was stopped after Bill summoned the Ristallian crater-hound Archie, whose simple brain couldn't be kept back. Sashana was arrested; a Traulian dampening helmet was used to block her psychic powers until she was put in a psychic-proof cell. (COMIC: Matildus)