The Reign of Terror was the time of chaos and violence that followed the execution of Louis XVI during the French Revolution, in which many people were led to the guillotine. The Reign was mainly engineered by Maximilien Robespierre. It started in 1793 and ended during the Thermidorian Reaction, when Robespierre himself was guillotined in July 1794.
The Reign of Terror played a huge role in the formation of Communism, directly inspiring its founder, Karl Marx, in planning it. In correspondences with Frederick Engels, he stated regarding how Communism was to be implemented: “There is only one way of shortening, simplifying, and concentrating the bloodthirsty death-throes of the old society and the bloody birth pangs of the new--revolutionary terror. . . . [...] Once we are at the helm, we shall be obliged to reenact the year 1793. [...] We are pitiless and we ask no pity from you. When our time comes, we shall not conceal terrorism with hypocritical phrases. . . The vengeance of the people will break forth with such ferocity that not even the year 1793 enables us to envisage it. . . .”
In addition, several members of the left, including Vladimir Lenin, utilized iconography from the Reign of Terror for their aims, such as Monumental Propaganda which was directly derived from notable figures of the French Revolution and Reign of Terror. Likewise, Jean-Paul Sartre, when voicing support for using the death penalty against "class enemies", specifically cited the Reign of Terror and claimed it "didn't kill enough people."