This is one of those stories from history that's well-documented enough but surprisingly isn't more prevalently known. If you know, you know, but let me try to set it up for those who might not...
It all started with a notorious inheritance scheme in 1600s France, where a noblewoman down on her luck murdered her father and two of her brothers slowly over the course of several months in order to rewrite the will in her favor. The woman in question, the Marquise de Brinvilliers, was soon accused of these murders, arrested, convicted after much torture, and eventually executed. While her story was considered quite scandalous, it wasn’t until after the death of her lover Captain Godin de Sainte-Croix that things started to escalate to something a little crazier. Post-death rumors soon started to circulate that the Marquise was actually testing out her poisoning skills on about 30 sick people interned at the hospital she worked at prior to offing de Brinvilliers père and all others who stood in her way. It was after this story came into the light that France went a little nutty for five years concerning all things to do with Ladies Who Poison.
This was where the infamous “Affair of the Poisons” began.
The story of this “affair” only gets stranger from there. Essentially it was the French equivalent of the Salem Witch Trials but it escalated to a particular kind of crazy since the investigators were able to tie it directly to the court of King Louis XIV. Imagine being told a history lesson by someone like Stefon from SNL: “Come down to Paris’s hottest new nightclub (adopts radio announcer voice) ‘Satan’s Hairy Teat!’ It’s got everything. Witchcraft, orgies, baby-killing, husband-killing, thumbscrews, those frilly things rich French guys used to wear…” When boiled down, it becomes notable as a case where a bunch of people (specifically aristocratic women) got accused of witchcraft and were then tortured horribly before burning at the stake/hanging/getting drawn-and-quartered.
You know - the usual good times we associate with that period.
It also turns out that a story as wild as this would obviously be popular for lots of people to write about. There are several books easily available (Anne Somerset's The Affair of the Poisons appears to be the key one), dedicated to going in depth about the detailed specifics of the Affair and all of the actors involved. Several websites have lengthy pieces about it too, especially ones focusing on the infamous fortune teller Catherine Monvoisin (“La Voisin”).
Hell, there’s even a video out there with a puppet about it:
"Puppet History" is quite a series of videos - mileage may vary but it's highly recommended if you enjoy this one.
I don't know if I can do this any more justice than others have but if you want more (aside from reading Somerset's book), here are some more articles to check out:
"The Scandalous Witch Hunt That Poisoned 17th-Century France" by Natasha Frost (Atlas Obscura)
"Sorcery, Sex and Murder: The Parisian 'Affair of the Poisons' Had It All" by David Roos (How Stuff Works)
"How The Affair Of The Poisons Uncovered A Network of Witchcraft" by Jen Jeffers (Ranker)
"La Voisin, France's Murderous Fortune Teller" by Jessica Toomer (SyFy)
"La Voisin and the Scandalous Affair of the Poisons" by Harald Sack (SciHi Blog)
"The Black Masses of La Voisin: How a Fortune Teller Became a Murderess in the French Royal Court" by Natalia Klimczak (Ancient Origins)
"La Voisin or a 17th century self-made woman à la française" by portiabridget (portable pieces of thought)