Complex sentence: She then descends into the same subterranean chamber where the Black Robe rite had taken place and reverses some of its effects at great cost to herself--she suffers a miscarriage that nearly kills her and ends her marriage to Arvath.
Split into simpler sentences: She then descends into the same subterranean chamber where the Black Robe rite had taken place. There, she manages to slow some of the ritual's effects, but at great cost to herself--she suffers a miscarriage that nearly kills her and ends her marriage to Arvath.