Complex sentence: The two stay together for nearly two hundred years, caring for Those Who Must Be Kept, before Bianca leaves Marius in Dresden, when she overhears him telling Pandora that he would leave Bianca if it meant having Pandora with him once more.
Split into simpler sentences: The two stay together for nearly two hundred years, caring for Those Who Must Be Kept, before Bianca leaves Marius in Dresden. She leaves because when talking to Pandora, Marius says that he would leave Bianca if it means having Pandora with him once more.