Hi! Bit of a weird one this. I'm looking into starting a D&D game. In the game, the players will play as demons, on Earth, where magic is fought against by the church. I'm looking to set the game around the time of the first crusade. I was hoping there might be some theological accounts of demons, "unexplainable events", satanic, pagan, or other cults, investigations into alchemy or magic etc.
Obviously the sources I'm looking for are specifically bad historical sources, which may mean this is a bit of a hard question to answer.
The game doesn't have to be entirely accurate, so if you can think of any of these things which don't occur in the Holy land in the 11th century but instead occur in rural America in the 1600s, or in Ravenna during the dark ages, I'm still happy to read about it to take inspiration.
This is quite a broad question but literally anything you can think of which I could use would be great. For example, the evidence used in the Salem witch trials and how they used "white magic" could be useful to me, as could the writings of Heinrich Cornelius Agrippa.
I'm not sure what you mean by "bad history" here--studying what medieval people thought and wrote about magic, demons, the devil, witchcraft, etc. is completely valid and often very interesting. And Christianity has quite a strong tradition of people thinking and writing about demons. In fact, I've often thought it would be really cool to read a fantasy story where medieval ideas about the cosmos and magic were real, and shaped events--I'm totally on board with your game.
If you want to make this easy, the single best resource is unfortunately anachronistic. The Malleus Maleficarum or Hammer of Witches is available online in English for free. It's the great 15th century witchhunting manual (the other major one, Johannes Nider's Formicarius, has not yet been translated in its entirety).
But if you want to go a little deeper:
Scholars often talk about the Middle Ages as a "magical world" in the sense that people saw the supernatural affecting the natural (and vice versa, to a certain extent) quite fluidly. From the early Middle Ages, we do see some evidence of good magic and bad magic. The First Crusade is right around a pivot point in Christian history where at least the scholars and pastors are starting to shift towards a view that all magic is maleficium--bad deeds, witchcraft. (The full diabolization of magic, as in only from the devil, really takes root later). But this is a little misleading from a modern standpoint, because high/later medieval Christianity still had plenty of "magical" elements in it. Crystal-gazing to predict the future, using prayers and lighting candles for treasure hunting, writing prayers on scraps of parchment and sticking them into amulets for protection, and all the visions of Jesus and saints that you could ever want occupied major places in mainstream Christian culture. Various regions had their versions of weather magic, too.
Some secondary books that might pique your interest:
If you're interested in Salem-related material, Increase Mather's Remarkable Providences is to a large extent a catalogue of ways that he believed demons were tormenting people in colonial Massachusetts--some of which are folkloric tales that were being told around the same time in England, set in England.