"It began in sunshine, not on a dark and stormy night."
- herein
THE SERVANTS OF TWILIGHT was first published in somewhat different form as TWILIGHT under the byline "Leigh Nichols" in 1984. The organization after which the book is named is a well-meaning religious sect who are determined to eliminate the anti-Christ before he does irreparable damage to the world. But "the anti-Christ", in this case, is a six-year-old boy, a sweet kid being brought up by his single-parent mom. Joey at first seems to be a random selection on the part of the Servants' leader, Grace Spivey.
This isn't one of Koontz' more comfortable books, incidentally, with a faceless bad guy. Koontz lets us into Grace's point of view quite a bit, and she isn't somebody who has deliberately chosen an evil path. She's a very troubled person, apparently a schizophrenic, and genuinely believes that Joey is the spawn of evil.
When the Servants begin stalking Christine and her son, nearly killing Joey's pet dog, the law can't protect Christine, as there isn't adequate proof that the Servants are responsible (or even that they're stalking her family). At that point, Christine takes action to protect her little boy by hiring a private investigator, Charlie Harrison. (Christine does her share of protecting the boy, but she's sensible enough to hire skilled help. Everyone has to sleep sometime.)
Most of the remainder of the book is an extended chase scene, although the object is to get away rather than to catch anyone. Several of Koontz' other books have this kind of structure; SERVANTS falls into the earlier versions' pattern, in which relatively isolated bad guys are chasing the good guys rather than vast conspiracies that require long-term solutions.
Charlie and Christine both come from troubled family backgrounds, like many of Koontz' protagonists (and Koontz himself), and have managed to become professionally successful. It's possible to draw a number of parallels between Koontz' own background and those of the main characters. While Koontz grew up with both parents, for instance, his father drank, so in a sense the author came from a single-parent family. In later life, Koontz' father was diagnosed as schizophrenic, like Spivey, but like Spivey had an ability to persuade people to follow his lead (though in his case, it was in matters such as starting new businesses rather than new religions).
Charlie's parents were abusive alcoholics, while Christine's demanding mother is herself caught up in religion that is more important to her than relating to her daughter or grandson. The protagonists are rather ambivalent about religion, on the whole, each having been devout at earlier periods in their lives, then having experienced a loss of faith. (Christine as a very young woman was a nun, her brother a priest, and their mother is still disappointed that Christine asked to be released from her vows. Christine's brother has since died.) Both Charlie and Christine are capable of intense loyalty and family feeling, while having parents that neither inspire nor deserve either.
The bad guys, on the other hand, are convinced that they're doing the right thing. As the story unfolds, the reader may even begin questioning Joey's situation, as many parallels begin accumulating that suggest that Spivey's random selection of the boy really may have been inspired by *something*, although the boy is still an ordinary (though very appealing) little kid. (He asks awkward questions, for instance, about where his father is, and didn't his father like him, and so on. Actually, his father gives Love At First Sight a bad name.)
Content warnings: Like a number of Koontz' books, SERVANTS contains one rather explicit sex scene (though not as explicit as that in WHISPERS), and quite a lot of violence, as the Servants seem to have few objections about killing people who get between them and "the anti-Christ". Organized religion has pretty much failed the protagonists (even if one discounts the rather large problem of a nominally religious organization coming after them with guns).
Still, this is a Koontz book. Bad things happen, some people are rotten, and organizations may fail to protect people properly, but individual good guys can manage to come through horrific episodes without being turned into monsters, even if they may suffer greatly in the process.