I think KevinSig is onto something. We know that Master Ferrus thought that Cavendish was dead. I believe he says as much when he first realizes that another etherealist is working against Spire Albion. We also know that as an etherealist grows in power, they develop "holes" in their sanity that they have to "fill" with something, and that eventually the "holes" become too big and/or too numerous that they "fall" through them, essentially having a mental breakdown where they cease to function.
Perhaps Cavendish had been deprived of her coping mechanism, like Folly's crystals or Ferrus's collection, when last Ferrus saw her, and she was "falling" to her death. I think that The Enemy is able to fill these "holes" with itself. It is putting itself into the pieces of her personality and mind that are missing. She appears sane (compared to Folly and Ferrus's endearing eccentricity), but her mind is not wholly human now. It sounds like some kind of partial possession to me.