Satanism
11th century Christians believed that Satan was a major factor in their world. So, in FW, he is. There are secret cults of Satanists practicing human sacrifice and witchcraft.
I will allow a PC to be a Satanist. I will also, however, allow Christian and pagan PCs to kill Satanic PCs for the experience points. So, this option is not recommended.
Satanism is not an original or creative religion. It is a dark mirror of Christianity, and exists only to mock it. Thus, everything Satanists believe or do is the opposite of what Christians believe and do. Since there is a Pope, there is an Anti-Pope. Since there is Mass, there is Black Mass. Christians avoid the seven deadly sins, so Satanists consider them virtues.