This is based on opinion. Please don't list it on a work's trope example list.
Nightmare Fuel / Turn Coat
Special mention goes out to the description of the Skinwalker in Turn Coat when Harry turned his Sight on the thing.
Try to imagine the stench of rotten meat. Imagine the languid, arrhythmic pulsing of a corpse filled with maggots. Imagine the scent of stale body odor mixed with mildew, the sound of nails screeching across a chalkboard, the taste of rotten milk, and the flavor of spoiled fruit.
Now imagine that your eyes can experience those things, all at once, in excruciating detail.
Made even worse by the fact that Harry will never, ever forget it. Three books later in Cold Days, his narration still stumbles every time he remembers the skinwalker.
On the subject of Shagnasty, there's what it did to Thomas to get to Harry. It slowly flayed him alive, then when his vampiric regeneration ran out and he was about to die, fed him a random innocent woman. Thomas's out-of-control Hunger would force him to rape them to death. Then it started over again, repeating the process so many times Thomas lost count. No wonder he fell off the Friendly Neighborhood Vampires wagon...
Even worse is the fact that Shagnasty only did that because it has Intellectus on how to torture you in the most horrific way possible. So it, in fact, has no idea why it tortured Thomas like it did, only that it would be the most effective way to cause pain for both Harry and Thomas.
Zero. Just...jesus. Try and read that passage when Harry describes walking in without shivering. You will fail. The White Court may not be as overt as the Red or Black Courts, but that just makes the times when their full depth is revealed to be that much creepier.
That's why the place was called Zero, I realized. Zero limits. Zero inhibitions. Zero restraint. It was a place of perfect, focused abandon, of indulgence, and it was intriguing and hideous, nauseating and viscerally hungry.
I felt a shudder run through me. This was the world as created by the White Court. This is what they would make of it, if they were given the chance. Planet Zero.
One gets the impression that Turn Coat was intended to de-humanize the White Court. After watching Thomas behave almost like a good person for several books, we're in danger of forgetting that the White Court vampires are ''monsters''. In one book we have Zero, Lara and her two sisters devouring three security guards who just risked their lives to protect the Raiths, Lara's stomach-turning defeat of Madeline, and finally Thomas's Face–Heel Turn.
The Senior Council discovering that their wartime decisions were all influenced by the traitor. Like, say, the Merlin realizing that he came a hair away from sentencing Molly Carpenter to death in a fit of pique. Or McCoy realizing that he escalated the war with the Red Court with his Colony Drop on Casa Verde. Or Listens-to-Wind realizing that he put political expediency ahead of saving an innocent man's life. God only knows what other decisions senior councilors made that could have been even worse. And that's nothing compared to the damage Luccio likely did while completely enthralled by Peabody.