Complex sentence: A boy named Menico arrives with a message of warning from Fra Cristoforo, and they seize him; but when they hear the alarm being raised by the sacristan, they assume they have been betrayed, and flee in confusion.
Split into simpler sentences: A boy named Menico arrives with a message of warning from Fra Cristoforo and they seize him. When they hear the alarm being raised by the sacristan, they assume they have been betrayed and flee in confusion.