The late 1800s is a perfect period, as it saw the rise of naive satanism, the Naughty Nineties period when Crowley and Encausse were in their heyday. An exotic touch with Mata Hari's eastern esotericism, "Castafiore" Calvet, Auguste Rodin, you have a full palette - dare you go as far as including the Rouen circuit (Cardinal Senator de Bonnechose/Bishop Billard/Maurice Leblanc) and the Pope himself? I would. The real world in those days was within a stone's throw of anything KK can dream up. But that's perhaps for the generation before, Conan Doyle (his gravestone speaks pages, off Wikipedia), EW Hornung (you can see who Raffles was based on), the birth of Spiritualism....one might even go back further in the Robert Louis Stevenson connection. Another angle which KK might find personally satisfying is the relationship between Doyle and Casement, together with the attack on the latter's mores by the British establishment, slap bang central to satanist creeds.
The logical next step is to investigate what happened in the late 1920s to get the FUDOSI satanists kicked out of France - they turned up in Belgium but with a very English flavour. Someone had a Hunting Group there for sure. Agatha Christie picking up Conan Doyle's gauntlet (her missing eleven days in 1926. for example, being an overused macguffin).