The Pendelton has a long and bloody history, but that bloody history extends into the past, beyond when the building went up. The hill it stands on is a site of horror and death; every 38 years, everyone on the hill is massacred in the space of a few hours, with few survivors. The group of people currently residing in the building have no idea what's in store for them. 
 ...And neither do we, really, until almost the end of the book. There are hints dropped, but if you don't pick up those hints, the entirety of what's happening isn't made clear until the last 10 pages. And, frankly, I'd be amazed if many people made it that far. 
 Apologies to Koontz (I really enjoy some of his other work), but this book is bad. The writing fluctuates between lovely and mush, the characters are two-dimensional, there are too many characters and the POV switches between them too quickly, and about halfway through the entire plot suddenly goes from horror to sci-fi with only a bit of warning--enough warning that I saw it coming and said, out loud, "oh, no. Oh, come on!" 
 While some of Koontz's descriptions are nicely written, in other places you get things like this: 
 In the fading light, he saw the snake-form fungi begin to throb, not every row in unison, but first some and then others. A wave motion, like the peristalsis that forced food down the esophagus and through the digestive tract, pulsed in these tubular organisms as though they might be swallowing live rodents or as if these were the intestines of a great beast. 
 Or the description of the first monster, that calls it a big-headed baby thing. 
 Not only are there too many characters (and most of them are too flat for me to care about), Koontz continues to introduce new ones until over halfway through the book--and two of them are literally only there to drop hints at us. Their conversation over dinner is what made the whole thing click for me, just pages before Koontz dropped the penny and upended all my expectations and understanding of what this book was (and not in a good wow-that's-clever way, either). 
 The most frustrating thing for me was that the central "monster's" motivations are only partially made clear. I get why it's doing what it's doing, but not why it has to scare the crap out of everyone in the building before it does it. (view spoiler)[It's essentially a machine, so why does it care so much that people are terrified before they're melted down and assimilated? (hide spoiler)] 
 My sister, who read this along with me, said it felt like Koontz had a bad acid trip or an Ambien-fueled nightmare and wrote it down verbatim without worrying about whether it made logical sense, and that's a really apt description of this whole mess.