Complex sentence: Romualdo starts to regain his memories but in doing so, it is revealed that a piece of the Black Witch has been hiding in his mind, and the witch appears and transforms Fantaghirò into a lowly creature off-screen.
Split into simpler sentences: Romualdo starts to regain his memories but in doing so, it is revealed that a piece of the Dark Witch has been hiding in his mind. The Dark Witch transforms Fantaghirò into a lowly creature off-screen.