Origins of the Witches' Sabbath by Michael D. Bailey

While the perception of magic as harmful is age-old, the notion of witches gathering together in large numbers, overtly worshiping demons, and receiving instruction in how to work harmful magic as part of a conspiratorial plot against Christian society was an innovation of the early fifteenth century. The sources collected in this book reveal this concept in its formative stages. The idea that witches were members of organized heretical sects or part of a vast diabolical conspiracy crystalized most clearly in a handful of texts written in the 1430s and clustered geographically around the arc of the western Alps.