The Day After Judgment - James Blish

The powerful black magician Theron Ware has opened the gates of Hell and unleashed an army of demons. Chaos and destruction prevail, and most of the world lies smouldering in ruins. But in a bunker beneath the ashes of Denver a small group of survivors are determined not to submit to the forces of evil. They summon the nuclear powers of the country's military arsenal to destroy Hell, but what can men do against the almighty evil that is Satan himself?

A direct sequel to Hugo Award winner James Blish's occult classic Black Easter (1968), The Day After Judgment (1971) can also be read as a standalone book. Like its predecessor, it features a blend of horror, science fiction, fantasy, philosophy, and black humor that makes it quite unlike anything else you have ever read.