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 very short novels form the third part of the thematic "After Such Knowledge" trilogy (title from T.S. Eliot's "Gerontion," "After such knowledge, what forgiveness?") with A Case of Conscience & Dr. Mirabilis. Black Easter was serialised as Faust aleph-null in If magazine.
