Complex sentence: She reveals her murderous husband's secret to her sister Anne, and both plan to flee the castle the next day; but, Bluebeard returns home unexpectedly the next morning and, noticing the blood on the key, immediately knows his wife has broken her vow.
Split into simpler sentences: She reveals her murderous husband's secret to her sister Anne, and both plan to flee the castle the next day. But Bluebeard returns home unexpectedly the next morning, and, noticing the blood on the key, immediately knows his wife has broken her vow.