I must begin by taking issue with some of the more negative reviews here. No, "Black Easter" is not a modern horror novel, or slasher film, or gore-fest. It was first published in serial form in 1968 (in "If" magazine) under the title "Faust Aleph-null." So, first things first, let's give this fine piece of writing its due ESPECIALLY since it is, as of this writing, forty-four years old. Some reviewers here have complained that it's not shocking now. Well, neither, I suppose, is Shirley Jackson's "The Haunting of Hill House," but it's a marvelously creepy ghost story nonetheless. And, like that book, give "Black Easter" a chance and it just might make your neck hairs stand up a bit.
The premise here is that an arms dealer contracts with a practitioner of black magic, the aim being to loose all the demons of hell upon the Earth for one night -- JUST TO SEE WHAT HAPPENS. What happens, unfortunately, is Armageddon; once loosed, the legion of demons cannot be whisked back to the underworld.
Some who've complained that "Black Easter" seems short and incomplete should know that there's a good reason for that: Blish considered it to be merely the first half of a novel, the other half being "The Day After Judgement." In 1990, a small publishing house called Gregg Press finally released the two novellas as the single novel Blish had intended, under the title "The Devil's Day." Presented in that way, both "Easter" and "Judgement" seem whole, and the entire story arc just makes more sense.
Both "Easter" and "Judgement" are worth reading. True, even for me they don't pack the punch they did when I first read them as a teenager in the 1970s. But still, there are some very dark, disturbing concepts here. Essentially, Blish is writing about the terrible price that experience and knowledge can extract from a person, even to the point of destruction.
And in fact, if the two books are considered as one book under the title of "The Devil's Day," then another of Blish's claims holds water: He considered this the final book of a trilogy about the price of knowledge, with the first novel being 1959's "A Case of Conscience" and the second being "Dr. Mirabilis," published in 1964. His title for the trilogy was "After Such Knowledge," a phrase pulled from the writings of T.S. Eliot.
Those first two books do not share the same character set that appears in "Easter" and "Judgement" or, if you prefer, "The Devil's Day." But if you're going to read "Black Easter," I would definitely pick up a copy of "The Day After Judgement" and read the two novellas back to back as a single novel, as Blish intended. They are fascinating books, their central question being, "What would happen if good were NOT necessarily stronger than evil?"
Finally, I do agree that Blish as a writer can be a bit problematic -- especially annoying is his tendency at times to tell rather than show. But in his best passages here, he presents a world both fascinating and repellent, an Earth awash in wickedness unleashed not with great malice but simply recklessness.