The Adversary Cycle is a series of six novels written by American author F. Paul Wilson. Originally known as "The Nightworld Cycle" (this name is even printed in the front of some early editions of Nightworld), John Clute, in his section on F. Paul Wilson's work in The Encyclopedia of Science Fiction, kept referring to "the Adversary." The author liked that better and so renamed the cycle.
The six novels that comprise The Adversary Cycle are (in chronological order):
However, it is worth noting that the book series does not fall into a strict chronological order, with alternative orderings suggested. For example, Wilson's website suggests an order of The Keep, Reborn, The Touch, The Tomb, Reprisal, and Nightworld.
The Tomb and The Touch were initially written as stand-alone novels; it is only in Nightworld that they were retroactively made part of the Cycle as their respective protagonists all come together to fight the final battle against the Otherness.
Although these six books make up the "core" of "The Adversary Cycle", many more novels and short stories tie into the series. For example, The Tomb is also the start of the Repairman Jack series of novels, which quickly became deeply entwined with the Adversary Cycle. They take place in the last three years before Nightworld and include some of the main characters from the original Adversary Cycle. This is all part of the "Grand Unification" theme of Wilson's work.