I know that accusing people of witchcraft and demon worship wasn't all that uncommon of a occurrence in the Middle Ages, but are there any examples of actual organized societies, cults or sects that were based around explicit demonic worship or occultism born out of the Christian (I guess in this case mainly Catholic) concepts of demons and devils?
To be a bit more specific, I'm thinking of something similar to modern forms of Satanism that are philosophically founded on being anti-Christian, as opposed to just being a non-Christian belief system that might have been deemed heretical, occultist or "devil worshiping" by Christian authorities.
(Note that I'm very aware that Crusader Kings and Paradox's other grand strategy games are several degrees separated from anything remotely resembling historical accuracy. Nonetheless, this particular element peaked my interest.)
I haven't played the game, but this sounds a lot like the fate of the Knights Templar.
Before I go into this story, let's make one thing clear: There is no evidence of organized Satanic ritual activity on any scale. I assume there are small groups of iconoclasts who paint themselves as faux "Satanists," and at various times in history people who practiced pagan religion were called "Satanists." But in the sense that you mean... No, it didn't happen.
Here's what did happen:
During the Crusades there existed a number of "Militant Orders" which were groups of crusading knights who lived a quasi-monastic lifestyle. The knights organized themselves in a manner similar to monks living in a monastery, and devoted themselves to both war and religious practice. One of the most prominent of these was the Knights Templar. The Templars were a powerful and wealthy order of knights whose influence waxed and waned throughout the Crusades, but by the end of it they were pretty well off, to say the least. Some go so far as to credit them with the invention of modern banking.
The King of France (Phillip IV, the Fair) wanted to get his hands on some of this wealth, and the easiest mechanism by which he could do this was to have the Knights tried as heretics. In 1307 he put forth a list of charges that included such things as: worshipping various Satanic beings and idols, taboo sexual practices, and performing certain acts that disgraced Christian symbols. The concept that they were a secret Satanist cult gave him the political and legal justification to abolish the order, execute the knights, and steal their stuff. It is virtually certain that these charges were fabricated.
It's all a rather fascinating (and tragic) story. These accusations and other invented stories about the Templars have led to them becoming something of a pop culture trope. Rumors of Templar mysticism or cults flourish in conspiracy theories or speculative writing, and you can see the influence of this in media such as "The DaVinci Code" and "Assassin's Creed" video games.