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Nightmare Fuel: Ghost Story
During Harry's invasion of the Big Bad's fortress from the Nevernever side, he unleashes the ghosts of killers on the enemy forces. Harry Dresden describes the scene as fresh scenes for his nightmares. He only describes the least disturbing sequence from the resulting slaughter, and leaves the rest up to the reader's imagination. It's bad enough.
And when you consider everything that Harry's described in previous books, it's cringe-worthy to imagine what he didn't want to tell us.
He Who friggin' Walks Behind. During a flashback, we finally learn the details of Harry's first encounter with it. Turns out it's a huge, incredibly strong and even more sadistic monstrosity that is always behind you, no matter where you turn, so the only way you can see it is in a mirror or other reflective surface. 16-year-old Harry first sees it reflected in an arcade machine in a gas station; he turns around to look and sees nothing, but when he turns back, it's now standing two feet closer. And smiling. Shortly afterward, it tore a person in three pieces with no effort at all. And no, you can't just stand with your back to the wall; Harry tried that, and it still came up behind him, lifted him by the neck with a tentacle, and threw him across the room. Think equal parts Shagnasty and Slender Man, and you'll have a bit of an idea.
Not to mention what Harry feels when looking at He Who Walks Behind. That descriptive monologue would have done HP Lovecraft proud.
Also, now we know exactly why Harry had such a Brown Note reaction in Blood Rites when he found out that He Who Walks Behind was the one behind the entropy curse.
The worst part of it wasn't seeing her in action so much as the way she described it. That it was easy to set up small illusions that would end with people killing each other. The same way it was easy to help Rosie and Nelson way back during the Proven Guilty casefile.
And Will describing the results of the intervention.
Will: I've got a picture in my head of being dragged off into the Nevernever by monsters that I still can't get rid of. When she gave it to me, all I could do was curl up into a ball and scream.
Harry's eventual fate if he failed to find his killer: to watch from the shadows as the people he cared about lived and died, and be powerless to do anything. That he's running around in his immortal soul during this only makes it worse.