In 1582 he was named judge in Bordeaux, and in 1608 King Henry IV commanded him to put an end to the practice of witchcraft in Labourd, in the French part of the Basque Country, where over four months he sentenced to death several dozen persons.
In his opinion, Satan had little sexual intercourse with single women, because he preferred married women for that implied also adultery, and the incest between mothers and sons at the end of the Sabbath was essential to give birth to demonic children, as well as a sexual act between a witch and a he-goat (believed to be Satan present at the reunion).
All these prejudices are reflected in his work Tableau de l'Inconstance des Mauvais Anges et Demons, published in 1612, not long after the process.
Maxwell-Stuart clarifies De Lancre's legal orientation on the evidence of witchcraft in Labourd: The confessions of male and female witches are in agreement with indicia so strong that one can maintain they are genuine, real, and neither deceptive nor illusory.
Thanks to these books we know something of what happened in the process that de Lancre directed against the people of Labourd, because the judicial records vanished during the French Revolution.
Maxwell-Stuart writes on De Lancre in his Witch Hunters that: ...L'incredulité et mescreance du sortilège plainement convaicue (1622), produced twelve years after his long personal engagement with witches and witchcraft, spends an impressive amount of learning upon showing that magic of any kind is not an illusion and should not be dismissed by those who are pleased to think otherwise.
This work aroused the ire of Gabriel Naudé, at one time physician to Louis XIII and later librarian to Cardinal Barberini, who in 1625 published a fierce response, Apologie pour tous les grands personages qui ont esté faussement soupçonnés de magie, to which De Lancre, duly irritated, replied two years later with his final work, Du sortilège.