Oh my.
I read a lot of fantasy, both good and bad, but I don't think this one even qualifies for bad. Arguably, this might be the worst fantasy book I've ever read. Saying why is hard, since it has so many issues, but I'll try to break it down a bit, highlighting the most critical ones, and skipping over the rest.
First off, some considerations about the author. I have already read something from Scott Ciencin. In particular, he was the writer of the first two books of the Avatar series (Shadowdale and Tantras), while the third one (Waterdeep) was written by Troy Denning, all three under the pen name Richard Awlinson. Well, the first two books were so terrible that they were almost incomprehensible, the plot was totally confusing, the characters were meaningless. All this, considering that the trilogy was so critical for the Forgotten Realms setting! The third book does a decent job of setting things a bit right, to the extent that it is possible. Now, just the fact that the third book was given to another author seems awfully indicative that TSR already knew that Ciencin had done a terrible job.
That said, this book has all the flaws of Shadowdale and Tantras. The story is confusing, things happen apparently at random. Things that happen are sometimes explained because the author doesn't know how to make the reader understand it in description (which would be fine at the time of Leiber or Howard, but not in the Nineties). The characters are as emotional as paper dolls, even though the story would offer a lot of opportunity for emotional development. Krystin - a street girl, essentially - meets Myrmeen and a few pages later she is hanging under the skirts of the woman she suddenly believes being her mother, with a surprising outburst of trust.
Trying to look from a wider perspective, those above are not the worst problems in this book.
The story is located in Calimport and its roundabouts, but for what it's worth it could be set in Waterdeep. There is none of the Calishite style, which had been already described in FR3 - Empires of the Sands. The city doesn't have anything different from a general city in the Heartlands. Probably it was set in Calimport to justify that there are no Harpers there, but the city of Calimport itself is not actually used as s relevant setting. Most of the fascination of the Forgotten Realms is that every place is quite unique, and this book fails miserably at that.
The Harpers plot as well is totally irrelevant. Apart from the occasional mention of Twilight Hall or Elminster, the connection with the Harpers is extremely loose, and the characters might be any group of goody-goody adventurers. There is no real reason to discomfort the Harpers in this book, other than having it fit into the Harpers series. In other words, seen from a broader perspective, this is not a book on the Harpers set in Calimport; it is a book on a group of adventurers in a large city, where the Harpers affiliation and Calimport were forcibly put in. It makes me think that Scott Ciencin had written most of this book as a separate project, and since it was not published as it was, it was remodeled by throwing it into some scarcely-known (back then, at least) place in the Forgotten Realms, sticking a Harper pin on the characters' breats, and voilà, someone at TSR approved it.
Finally, concerning the genre. This book is more some sort of fantasy-horror story than pure horror. It is probably influenced by They Live and some conspicuous sci-fi literature about "the monsters that mingle with the normal people" (the same line that later became Men in Black). Setting this kind of horror story in the Forgotten Realms, in general, is not a brilliant idea. The Forgotten Realms are more suited for heroic or epic fantasy. However, if the author is very good, such a story might be groundbreaking, a masterpiece of inventive, turning the roots of the Forgotten Realms setting upside down in a captivating story.
Unfortunately, the author of this book is not up to the task.