https://en.wikipedia.org/wiki/Catherine_de'_Medici#Links_to_the_occult

Catherine de' Medici has been labelled a "sinister Queen… noted for her interest in the occult arts". To some, Catherine and Henry's inability to produce an heir for the first ten years of their marriage gave rise to suspicion of witchcraft. Labouvie suggested that women's power was believed to be the ability to create and sustain life, whilst witches were believed to have the opposite power; that of attacking health, life and fertility. An infertile woman, and in particular an infertile queen, was therefore regarded as 'unnatural' and a small step from supernatural. Elizabeth I was treated with similar suspicion—she too entertained questionable characters (such as her advisor, John Dee), and produced no official heir. Essentially, however, there exists no concrete proof that either woman took part in the occult, and it is now believed that Catherine's trouble in providing an heir was in fact due to Henry II's penile deformity.

Suspicion was fuelled to some degree by her entertainment of questionable characters at court—for example, the reputed seer Nostradamus, who was rumoured to have created a talisman for Catherine, made from a mixture of metals, goat blood and human blood. Catherine also gave patronage to the Ruggeri brothers, who were renowned astrologers, but were also known for their involvement in necromancy and the black arts. Cosimo Ruggeri, in particular, was believed to be Catherine's own "trusted necromancer, and specialist in the dark arts", although there is not a great deal of surviving documentation to tell of his life. Though some suggest that they were simply magicians, for many living in Italy at the time, the distinction between 'magician' and 'witch' was unclear. Entertaining individuals that appeared to subvert the natural religious order during the most intense period of witch hunting and a time of great religious conflict was therefore an easy way to arouse suspicion.

Catherine herself had been educated in astrology and astronomy. It has been suggested that Catherine educated her son, Henry III, in the dark arts, and that "the two devoted themselves to sorceries that were scandals of the age". As a result, some (more extreme) authors believe Catherine to be the creator of the Black Mass, a Satanic inversion of the traditional Catholic Mass, although there is little to prove this aside from Jean Bodin's account in his book De la démonomanie des sorciers. Nevertheless, Catherine was never formally accused or prosecuted despite the fact that her reign experienced the greatest number of prosecutions for Witchcraft in Italy. This lends some weight to the suggestion that people were labelled 'witches' simply because they did not act the way a woman would have been expected to act, or simply to suit personal or political agendas. This may be particularly true for Catherine as an Italian woman ruling in France; several historians argue that she was disliked by her French subjects, who labelled her "the Italian woman". In any event, the rumours have made a mark on Catherine's reputation over time, and there are now many dramaticised works about her involvement in the occult.