Black Easter and The Day After Judgment were written with the assumption that the ritual magic for commanding demons, as described in grimoires, actually works.

In the first book, a wealthy arms manufacturer comes to a black magician, Theron Ware, with a strange request: he wishes to release all the demons from Hell on Earth for one night to see what might happen.

Tension between Ware and Catholic white magicians arises over the terms and conditions of a covenant that provides for observers and limitations on interference with demonic workings.

Black Easter ends with Baphomet announcing to the participants that the demons cannot be compelled to return to Hell: the war is over and God is dead.

It has been suggested that Blish got the name for his black magician from the titular character in Harold Frederic's 1896 novel The Damnation of Theron Ware.