Nightworld is the sixth and final volume in a series of novels known as The Adversary Cycle written by American author F. Paul Wilson. First published in 1992 by New English Library in England (May) and Dark Harvest in US (August). Nightworld completes The Adversary Cycle, which consists of six books: The Keep, The Tomb, The Touch, Reborn, Reprisal, and Nightworld. Nightworld was heavily revised for its reprinting in 2012 to include the events that happened in the subsequently written novels in the Repairman Jack series.