What Dracula and King's Salem's Lot is to vampires, Black Easter is to demons. These days, vampires, witches and wizards are lovable creatures in fiction. There are even neo-Pagans today who embrace a belief in ceremonial magic as benign. Blish says in his Author's Note that every one of the "novels, poems and plays about magic and witchcraft" he's read treat it as "romantic or playful." He sought to write a treatment that "neither romanticizes magic nor treats it as a game." His book is dedicated to C.S. Lewis and even includes an extensive quotation from his Screwtape Letters heading one of the chapters. So although I'm not sure I'd classify this as out and out Christian fiction, this does come out of that world view and takes the demonic seriously--that's what does make it unusual and at times fascinating. It's obvious not just from his note but the vividness of his details and even the quotes heading chapters Blish did extensive research--actually reading grimoires and manuscripts on ceremonial magic. Blish is best known as a science fiction author, and he gives his magicians a grounding that really makes if feel as rigorous as any science--and it's obviously an allegory for amoral science and its destructive power. The world building in that regard is first class, which is why I'm not rating it lower. The characters not so much--and the plot feels a bit thin. I never found it scary, nor was scared for the characters, and the ending, which I thought predictable, didn't have much impact upon me. Also, I had read this before--albeit over a decade ago--but it didn't make enough of an impression to really be memorable--thus why I'm not rating it higher.