This is based on opinion. Please don't list it on a work's trope example list.
Nightmare Fuel / Storm Front
The way that the mobster and his girlfriend are murdered in Storm Front. Their hearts EXPLODE OUT OF THEIR CHESTS. While having sex.
Although this can double as a horrific Crowning Moment of Heartwarming— you later find out only the girl was targeted. The mobster dies because he was in love with her, and in that moment, their hearts were as one.
The realization of what the spell is supposed to do when properly powered. The version of the spell used in this book is roughly the equivalent of powering something through a potato battery.
For those who haven't read Changes It is a genocide spell. It starts with the target and starts traversing their family tree. Killing EVERYONE along the way. Yeah...
Which raises a much, much more horrifying possibility than what's suggested above, as to why Tommy Tomm died along with Jennifer Stanton: he might have gotten her pregnant a few months earlier. Spell strikes Mom-to-be, jumps to unborn baby, then jumps to Daddy, all of them within the few feet of range the spell had in its potato-battery variant. The stricture that the heart-ripper curse spares the youngest wouldn't apply, if the rabbit Sells sacrificed for his ritual was a juvenile he'd gotten from a pet store.
While Butters was 'safely' behind The Masquerade at this point, he still spent years trying to figure out what was going on after examining those corpses. When the subject comes up during Cold Days, one gets the impression that he's kind of relieved to finally have an explanation. Less relief that it came up out of a discussion of what the bad guys might be able to do with the blood Harry was leaving behind.
The cat-sized and still-growing scorpions - Primal Fear at its best.
Subsequent books make it clear that most vanilla mortals who encounter the supernatural have to block it from their memory in self-defense, because it's too psychologically destabilizing to cope with. The ThreeEye addicts don't have that respite available to them, because things experienced with the Sight cannot be forgotten. Ever. They're never going to get over what they've Seen.