Black Easter

Black Easter is a fantasy novel by American writer James Blish, in which an arms dealer hires a black magician to unleash all the demons of Hell on Earth for a single day. The novel initially depicts the assassination of a Governor of California by a black magician working as a contract killer. The same magician is then hired to release every demon in Hell for a brief time period. However, the demons cannot actually be returned to Hell by the end of the novel. Their traditional opponent, God, has already retired and nobody else can compel them to return to Hell.