I’ve found an excellent article on Wikipedia about the Nephandi. Nephandi are twisted mages serving unnameable horrors. I’ve always found that the Nephandi were a beautiful adaptation of HP Lovecraft‘s work. His horrors of the Cthulhu Mythos are the ones that the Nephandi worship.
I have most of the Mage: The Ascension books at home and I have always loved that RPG system.
One of the reasons I’m researching the Nephandi is for my story Symria. I’ve made a few corrections to the article and removed a few wiki links, but it’s mostly intact from its entry on Wikipedia.