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How did the Cinq-Mars conspiracy threaten Richelieu near the end of his life?

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Full-length portrait of Cardinal Richelieu by Philippe de Champaigne, showing him in red clerical robes holding a red book, standing beside a balustrade with a draped curtain behind. — Cardinal Richelieu As both statesman and churchman, Cardinal Richelieu was the acknowledged architect of France's greatness in the 17th century. In 1640, when this full-length portrait was painted (likely by Philippe de Champaigne), he was embroiled in a dispute with the pope and other religious controversies along with many other consequences of expanding French power across Europe. He would die two years later, facing conspiracies against his life and suffering ill health.

The conspiracy led by Henri Coiffier de Ruzé, marquis de Cinq-Mars, posed a significant threat to Cardinal Richelieu near the end of his life by aiming to remove him from power and influence.12

Cinq-Mars, a favourite of King Louis XIII, recognized that the Cardinal intended to block his political ambitions and subsequently plotted to eliminate him. The conspiracy involved high-ranking figures, including the King’s brother, Gaston, Duke d’Orléans, and planned to incite revolts and open French frontiers to the Spanish forces with whom France was at war. On March 13, 1642, Cinq-Mars signed a secret treaty with the Spanish king Philip IV, securing promises of military aid for the rebellion.13

Richelieu’s secret service eventually intercepted a copy of this treaty on June 11, 1642. The exposure of the plot led to the arrest of Cinq-Mars two days later, his subsequent conviction for treason, and his execution by beheading. Despite his failing health, Richelieu successfully navigated this final crisis, maintaining the King's support until his own death later that year.123