When I was writing Hell Spawn for Silver Empire, I knew it would be something horror based by the time I was done with the first corpse.
I didn't know it was actually scary until I got Tommy Nolan home.
If you haven't read the book, at this point, our Detective has been working his butt off on the murder inquiry for over a day. He hadn't gotten sleep for about 48 hours by the time he went back home to collapse.
Then he came home and discovered that his son's best friend in school had been murdered.
Yes, that was Nolan's victim.
Then night fell, and everyone tried to go to bed.
Then it REALLY got dark.
I will not go into all of it here, but I can tell you that as part of the writing process, I did some research. Hell Spawn went as dark as it did because I had researched serial killers in high school, and demonic possession and infestation for this book. For the record, look up John Douglas for serial killers, and demonology included Father Amorth and Deliver Us from Evil (the book, not the lame Eric Bana film).
By the time I was done with the scene, I think my immediate email to my publisher was "I know you wanted UF (Urban Fantasy) but I think I'm writing a horror novel."
Thankfully, Russell liked it anyway.
But after that, the entire novel got dark. There were times I had to remind myself that the scene was written in daylight. It felt like this freaking demon was everywhere.
And all it wanted to do was kill Nolan.
Confronting the demon in its lair became interesting on a pure human level. It knew things it shouldn't. It turned furniture into deadly weapons.
Then there's the Rikers Island riot. But that's another story.
From there, I felt like the series settled down, but with the occasional dark moment where you thought, "Well, that's not good." Granted, I may have a warped point of view. I know what's in the dark. Perhaps zombies with automatic weapons are scary to some people. Perhaps going one one one with a Terminator-like bokor might be creepier than I had in my head at the time.
... Maybe I shouldn't say the story settles down, but that the sense of omnipresent dread is gone for Death Cult and Infernal affairs (books 2 and 3).
I decided we needed it back for City of Shadows.
... Okay. By the end of Infernal Affairs, the showdown with the ultimate villain involved some cool ideas that hadn't occurred to me before. So I made it the element of an entire novel.
Because I can, so there. But a little ombramancy never hurt anyone.
Oh, it's absolutely *lovely* when you get to put your research into your fiction...! :D