She first came to police attention in 1677 as a practician of occult magic with renegade priests, associated with Louis de Vanens, and La Voisin.
In 1679, Montespan hired La Voisin to murder the monarch and his latest love interest Marie-Angelique, Duchesse de Fontanges.
When the plans failed due to the arrest of La Voisin, Montespan hired Filastre to perform the murder on Fontanges.
On 30 September the Chambre Ardente, acting on the evidence given, condemned Filastre to death, before which she was to be subjected to further intensive interrogation of "the extraordinary question".
To prevent Filastre's testimony being read aloud in court, Louis XIV suspended public hearings of the Chambre Ardente.