Cinq Mars, Henri Coëffier Ruzé d'Effiat, marquis de, 1620-42, French conspirator. Introduced at court by Cardinal Richelieu at an early age, Cinq Mars rapidly rose in King Louis XIII's favor and was made master of the horse. He joined in a conspiracy with Frédéric Maurice de Bouillon and Gaston d'Orléans against the cardinal. The discovery of a secret treaty they had signed with Spain led to their arrest, and Cinq Mars and his friend, François de Thou, were executed. The conspiracy formed the basis of Alfred de Vigny's novel Cinq-Mars and Gounod's opera of the same name.