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.

In 1673, when the King's interest in Montespan seemed to wane, Montespan again employed La Voisin, who provided a series of black masses officiated by Étienne Guibourg.
On at least one occasion, Montespan herself acted as the human altar during the mass.
La Voisin also provided Montespan with an aphrodisiac, with which Montespan drugged the King.
During the King's affair with Madame de Soubise, Montespan used an aphrodisiac provided by Voisin's colleague Françoise Filastre and made by Louis Galet in Normandy.

In 1677, Montespan made it clear that if the King should abandon her, she would have him killed.
When the King entered into a relationship with Angélique de Fontanges in 1679, Montespan called for La Voisin and asked her to have both the King and Fontanges killed.
    La Voisin hesitated, but was eventually convinced to agree.
At the house of her colleague, Catherine Trianon, La Voisin constructed a plan to kill the King together with the poisoners Trianon, Bertrand and Romani, the last being the fiancé of her daughter.
    Trianon was unwilling to participate and tried to make her change her mind by constructing an ill-fated fortune for her, but Voisin refused to change her mind.
The group decided to murder the King by poisoning a petition, to be delivered to his own hands.

On 5 March 1679, La Voisin visited the royal court in Saint-Germain to deliver the petition.
That day, however, there were too many petitioners and the King did not take their petitions, which foiled her plan.
Upon her return to her home in Paris, she was castigated by a group of monks.
She handed the petition to her daughter Marguerite Montvoisin and asked her to burn it, which she also did.
The next day, she made plans to visit Catherine Trianon after mass, to plan the next murder attempt upon Louis XIV.
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.
In 1673, when the King's interest in Montespan seemed to wane, Montespan again employed La Voisin, who provided a series of black masses officiated by Étienne Guibourg. On at least one occasion, Montespan herself acted as the human altar during the mass. La Voisin also provided Montespan with an aphrodisiac, with which Montespan drugged the King. During the King's affair with Madame de Soubise, Montespan used an aphrodisiac provided by Voisin's colleague Françoise Filastre and made by Louis Galet in Normandy.
In 1677, Montespan made it clear that if the King should abandon her, she would have him killed. When the King entered into a relationship with Angélique de Fontanges in 1679, Montespan called for La Voisin and asked her to have both the King and Fontanges killed.     La Voisin hesitated, but was eventually convinced to agree. At the house of her colleague, Catherine Trianon, La Voisin constructed a plan to kill the King together with the poisoners Trianon, Bertrand and Romani, the last being the fiancé of her daughter.     Trianon was unwilling to participate and tried to make her change her mind by constructing an ill-fated fortune for her, but Voisin refused to change her mind. The group decided to murder the King by poisoning a petition, to be delivered to his own hands.
On 5 March 1679, La Voisin visited the royal court in Saint-Germain to deliver the petition. That day, however, there were too many petitioners and the King did not take their petitions, which foiled her plan. Upon her return to her home in Paris, she was castigated by a group of monks. She handed the petition to her daughter Marguerite Montvoisin and asked her to burn it, which she also did. The next day, she made plans to visit Catherine Trianon after mass, to plan the next murder attempt upon Louis XIV.
La Voisin