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Blurb

Black Easter is a Nebula Award-nominated fantasy novel by James Blish in which an arms dealer hires a black magician to unleash all the Demons of Hell on earth for a single day. It was first published in 1968. The sequel is The Day After Judgment. Together, those two short novels form the third part of the thematic "After Such Knowledge" trilogy with A Case of Conscience and Doctor Mirabilis. Blish has stated that it was only after completing Black Easter that he realized that the works formed a trilogy.
A shorter version of Black Easter was serialized as Faust Aleph-Null in If magazine, August–October 1967; the book edition retains the phrase as its subtitle. Black Easter and its sequel were later published as a single volume under the title Black Easter and The Day After Judgement; a 1990 edition from Baen Books was renamed The Devil's Day.