This is based on opinion. Please don't list it on a work's trope example list.
Nightmare Fuel / Blood Rites
Madge, after the remorseless entropy curse is redirected at her.
''She had plenty of time to feel it as the demonic killer, the guiding mind who had been behind the entropy curse, flowed in its semigaseous form into her mouth and throat and lungs, then extruded savage spines and tore her apart from within.
Madge didn't manage to get out a scream as she died.
But it wasn't for lack of trying.''
Lord Raith. Just entirely. Damn pretty-boy vampires he is terrifying.
Harry's description of the Black Court thralls and Renfields with The Sight:
I don't know if you've ever seen a sheep slaughtered for mutton. The process isn't fast, even if it isn't really cruel. They make the sheep lie down on its side and cover its eyes. The sheep lies there without struggling and the shepherd takes a sharp knife and draws a single neat line across its throat. The sheep jerks in a sharp twitch of surprise, while the shepherd holds it gently down. It smells blood and stirs more. Then the animal quiets again under the shepherd's hand. It bleeds. ...
In the first battle with the Black Court vampires, two of them start feeding off Lara by chewing holes in her breasts. Nausea Fuel and terrifying.
The curse itself: It's directed at the women around Genosa, all of them completely blameless, and most of them die horribly because of harmless things: one woman is water-skiing and gets hit by a car that goes off a bridge; one goes to get some golf-clubs out of a car and gets stung to death by thousands of bees; one takes a shower, the water turns scalding hot and she falls through the glass into a pool of water that becomes electrically charged; and the last goes looking for a bottle of water and ends up walking right into the firing line when a gun goes off and misses its intended target.
What Lara does to Lord Raith. Yes, he deserves it (and then some) but the description of Lara shucking her clothes and preparing to rape her father both physically and mentally into total submission is chilling, and Harry specifically states that if he, Murphy and Thomas had stuck around a few seconds longer, they probably would have lost their minds.
The damage Harry's hand suffers in the raid on the Black Court Vampire base. He notes blisters growing on it in the face of the heat of the flame throwers, his fingers appearing to grow thinner and curl as the flesh seemed to melt off them like wax, and that the pain made him momentarily blind. It's several books before the hand starts functioning anywhere near the degree that it originally did.
Ebenezer McCoy: cool old guy, Badass Grandpa... wielder of the Colony Drop via satellite.... And he's revealed to be "The Blackstaff", the one man allowed, and encouraged to, break the Laws of Magic in defence of the White Council. And Changes shows that the staff itself allows him to ignore The Corruption from breaking the Laws of Magic. Given what he can do with just a satellite, and he's self-admittedly responsible for The Tunguska Event, Krakatoa, and the New Madrid earthquakes, what could he do if he weren't so moral and the asteroid belt were closer?
When Harry realizes that Lord Raith is responsible for his mother's death, he just about goes crazy with rage, in a way we haven't seen yet. Ebenezar calmly reaches out and grabs Harry by his injured hand to make him calm down, and it's only when he does that Ebenezar reveals that he's tried to kill Raith three times already, and none of his attempts worked.