The Reign of Terror during the French Revolution was led by Jacobin leader Maximilen de Robespierre. He took control of the military and police force under the Committee of Public Safety and the Revolution's governing body, the National Convention, and used these forces to purge people who disagreed with him.
Starting in September 1793, Robespierre identified, arrested and beheaded Revolutionary leaders who disagreed with him or who weren't sufficiently radical or conservative. Between 15,000 and 50,000 people were executed in 11 months. Robespierre's downfall began when he proposed to eliminate Christianity from France and replace it with a new set of morals. On July 27, 1794, a group of Jacobins arrested him. He was guillotined the following day.