The Otherness is engaged in battle with a force known as the Ally for control over existence. Bear in mind that while the Otherness is inimical to human life, the Ally is not benevolent to humanity. The Ally may utilize humans as tools, but it does not oppose the Otherness for the benefit of humanity. On Earth (and in Wilson's novels), both the Otherness and the Ally use varying types of champions to further their respective goals.
On Earth, the primary champion of the Otherness is Rasalom. Rasalom is older than human civilization as we know it, coming from the mythic First Age, a time before recorded history when magic was commonplace. A powerful creature who may have been human at one point, Rasalom is featured several of Wilson's novels, including The Keep, Reborn, Reprisal, Nightworld, and the Repairman Jack novels Conspiracies and Gateways. Rasalom is typically depicted as male, but he is capable of changing his appearance, including his apparent gender. He favors using anagrams of his name, such as Molasar, Sara Lom, Rafe Losmara, etc.
Rasalom makes his first appearance in The Keep. He at first appears to be some sort of entity trapped in the titular structure, and for much of the novel fools everyone (including the reader) into thinking he is a vampire. Later, his longtime nemesis (and champion of the "other force") Glaeken arrives at the Keep and seemingly destroys the beast. However, Rasalom is a tricky one, and his death here does not hold him back for long.
Reborn, the fourth book in the Adversary Cycle, takes place in 1986. A young man named Jim Stevens discovers that he is in fact a clone. The entire cloning process, which took place in 1941, was initiated and aided by the Otherness, and Jim's conception took place at the exact moment that Rasalom was apparently defeated. Being a clone, Jim is no more than a soulless "shell" in which Rasalom's spirit resides, and upon impregnating his wife, the Otherness does everything it can to keep her safe for the delivery of her baby: Rasalom, reborn!
Following the timeline of the books, Rasalom appears in two Repairman Jack novels, Conspiracies and Gateways. In Conspiracies, he is under the guise of Sal(vatore) Roma, the head of a conspiracy theory convention called SESOUP (the Society for Exposure of Secret Organizations and Unacknowledged Phenomena). Aided by an Otherness entity (in the guise of a monkey, referred to only as Mauricio), "Sal Roma" is still in a state of uncertainty as to his "rank" with the Otherness.
In Gateways, a Repairman Jack novel, Jack confronts Rasalom, but is completely powerless against the creature. Despite his instincts telling him to kill Jack right then, Rasalom spares Jack's life and warns him of terrible things that will come into his life.
Rasalom appears again in Reprisal, tormenting the life of former priest Bill Ryan (a major character in Reborn) as he comes to full power. Glaeken (from The Keep) also appears here, but because the other force mistakenly thought that Rasalom had been defeated, all of Glaeken's powers, namely his lack of aging, have disappeared, and now he is an old man. Upon discovering this, Rasalom feels that there is nothing to stop him from taking over, and so the final book in the Adversary Cycle follows immediately.
Nightworld is Rasalom's final play, and he attempts to redesign Earth, creating a type of Hell that has a perpetual night, and countless portals to the hostile dimension of the Otherness. Since this book was written back in 1992, there have been nine new Repairman Jack novels which have been giving him a whole slew of experience and knowledge of the Otherness, and thus the original form of Nightworld is inconsistent. F. Paul Wilson has been revising much of the original text to accommodate for these "updates," and the revised edition will come out in later 2006.