The most important client of La Voisin was Madame de Montespan, official royal mistress to King Louis XIV of France. Their contact was often performed through the companion of Montespan, Claude de Vin des Œillets. Montespan was alleged to have hired La Voisin in 1667 to arrange a black mass. This mass was celebrated in a house in Rue de la Tannerie. Adam Lesage and abbé Mariotte officiated, while Montespan prayed to win the love of the king. The same year, Montespan became the official mistress of the King, and after this, she employed La Voisin whenever a problem occurred in her relationship with the King.