Maximilien Robespierre played a significant role in the French Revolution, primarily known for starting the Reign of Terror. During this period, which lasted from 1793 to 1794, he was a leading figure in the Committee of Public Safety and was instrumental in implementing policies that led to the execution of thousands of perceived enemies of the revolution, including King Louis XVI and Queen Marie Antoinette. Therefore, the correct answer is:
He started the Reign of Terror.
Robespierre was not involved in defending the monarchy, leading the attack on the Bastille, or writing the French constitution.