The Silent sees Missi slowly going mad, being all alone in the Castle after Duke left. It is probable that this change in mind is not her situation alone, but mostly caused by the Staff. Fueled by the corruption the Staff causes she becomes more and more paranoid about Duke returning to the castle.
This is manifested in her being chased by shadow creatures, as well as the shadow of duke himself and her imagining seeing Dukes face in paintings. After seemingly getting consumed by her own fears and madness she manages to throw the Staff away from her, immediately stopping the hallucinations as well as turning her back into a human.
Realising what happened she takes her things and prepare to leave the castle. Moments before leaving completely she spots Duke standing in the courtyard of the castle. Gripped by her hatred for Duke, she takes the staff, rips off her glasses and destroys her backpack against a wall, right before stepping outside to find an invitation from Duke