Complex sentence: Becoming a fanatic Huguenot, he determined to kill the Duke of Guise and, pretending to be a deserter, gained admission to the camp of the Catholics who were besieging Orléans.
Split into simpler sentences: Becoming a fanatic Huguenot, he determined to kill the Duke of Guise. Pretending to be a deserter he gained admission to the camp of the Catholic army that was besieging Orléans.