Where the Evil Dwells is set in an alternate timeline where in late antiquity Central Europe was swarmed by a horde of ogres, trolls, fairies, dragons, harpies, and other beasts collectively known as the Evil. Some of the Evil are more beastly than others, but altogether they are not a divided band of miscreants, but seem to have some sort of a confederate structure and are able to present a united front to the Romans. Initially this was bad news for the empire, but ironically the presence of the Evil far from hastening its downfall, encouraged the Romans to put their squabbles aside and unite against the common threat. Moreover, the Evil served as a buffer state between Rome and the incursions of the steppe peoples, so the empire never had to deal with the barbarian invasions of the Volkerwanderung. As a result the empire managed to survive more or less intact until the date of the book, which I've either missed or is never mentioned explicitly.
However, none of this really matters. The story is about Charles Harcourt and his motley band of friends who plunge into the Empty Lands (where the Evil dwells) to retrieve a prism that houses the soul of a saint. It could have been set in alt-history England, France, Germany, Poland, or any high fantasy setting without changing anything about the plot or, indeed, the setting. The only Roman thing in the book is a single character who pops up at the start of the book, leaves, then shows up again near the end only to promptly die. This highlights a reoccurring problem of the book -- it is very much governed by the Rule of Cool. The author throws in elements that tickle his fancy, realises he doesn't know what to do with them, forgets about them and carries on.
One of Charles' companions is the Knurly Man, who is not human. What he is, we don't know. Where he comes from, we don't know. Why he is questing with humans, we don't know. Why humans didn't mistake him for one of the Evil and burn him, we don't know. What does his non-humanness bring to the plot? At one point he procures a healing salve to treat a character's wounds, and once or twice he sheds some light on the backstory of some of the denizens of the Empty Lands, which amounts to "This thing is very ancient, and I don't know much about it".
Soon after the party enters the Empty Lands an earth elemental approaches their camp. A character squashes it. The Knurly Man says this was a terrible idea, as the elementals are more ancient than the Evil itself and this will no doubt bring calamity upon them. They enter a swamp to escape the elementals and they are never mentioned again.
At the journey's end they find Charles' long lost love, who had become evil and tries to claw the eyes out of Charles' new girlfriend. Charles pushes her away, she screeches, and scuttles off. Why she became evil, what was her endgame, and what happened to her after, all a mystery.
None of these elements were added with an overarching goal in mind (and there are a lot more). I imagine the reasoning of the author was, "A party of humans is boring, we need something gnarly. Gnarly? I like the way I think", "If we have the party intercepted by the Evil now, then I'll have to come up with a convincing reason for why they were able to escape. How about they get intercepted by something else? But what else lives here? Let's just say it's something very old", and "Wouldn't it be cool if instead of running to welcome Charles, Eloise was evil instead? Bam". I'm not a strict adherent of Chekhov's gun, but I do believe in writing with intent. Introducing an element that does not contribute to the plot, setting, or development of themes, that you don't intend to develop or explain is sloppy writing. That's how the super awesome stories I wrote with my best friend when I was twelve looked. To be fair, the author at least has the self awareness to lampshade pretty much all such elements, but lampshading is more effective if you do it rarely enough for the reader to forget about it.
On the flipside to elements which do nothing, there are the elements that make the entire plot possible, but are also dropped in with little or no explanation (or explanation that is left until the last few pages of the book), and there are plenty of these too. The party meets a wandering wizard, who later turns up to save them on a couple of occasions; four wooden gargoyles save them from an attack of the Evil and escort them for the rest of their journey; when they are lost and surrounded by the Evil they are literally teleported to their destination by what is apparently the soul of the saint and/or some wizards who were friends with the saint? These are not quite Deus ex Machinae, except perhaps for the last, as there is at least some foreshadowing, and ex post exposition dumps, but they are still very unsatisfying. Charles Harcourt is the star of this quest, but he wouldn't have gotten anywhere without lightning bolts coming down from the sky to rescue him whenever he messes up, which he does a lot.
What redeems this book is a few passages of highly effective writing. The highlight is definitely when the party enters a temple to escape a storm and finds themselves beset by what appear to be Elder Gods. The scene is masterfully set, from the fury of the storm outside, to the eerie quiet of the temple proper, to the sudden nightmareish cacophony of things that should not be threatening to burst in and consume them. Of course, none of this is relevant to the rest of the plot, the characters are saved by some magic none of them can properly explain, and once out they promptly forget about it and move on.
I don't know if this book is meant to be part of a series or set in some wider universe, but taken on its own merits it's a confused mess with a few flashes of brilliance.