This is based on opinion. Please don't list it on a work's trope example list.
Nightmare Fuel / The Belgariad
Polgara is explaining to Ce'Nedra about herself (paraphrased):
Polgara: In the prophecy we serve, I am Polgara the Sorceress, daughter of Belgarath and great-aunt of Garion. Ce'Nedra: And in the other one? Polgara: In the other one, I am the Bride of Torak. Ce'Nedra: (gasps) Polgara: Now you understand. I've been terrified of Torak ever since my father explained that to me, when I was around twelve. Ce'Nedra: But surely you could resist him. Polgara: You don't understand, Ce'Nedra. He's a God. Not even my power can withstand him, and if I fall to him, then I'll probably be deliriously happy to be his bride, but there will be a part of me that will be screaming in horror — and will keep screaming until the end of time.
To recap: She's the prophecied bride of an insane God who will, if he can overwhelm her, brainwash her to love him and probably force her to fight her friends. And this is the same god who ordered his priests to sacrifice a person an hour by cutting out their hearts, and whose entire left side is currently burning. Yeah.
Either book four, Castle of Wizardry, or book five, Enchanter's End Game. Those with a domination phobia should not read.
What Belgarath does to Zedar in the end.
And then the prequels reveal what Zedar did over the course of his life, which includes driving two of his sworn brothers to suicide via self-obliteration and betraying a truly innocent woman to a horrible death. Yeah, there's a reason Beldin's said that if Belgarath ever undoes what he did, he'll do it again.
The "smiley plague" from Demon Lord of Karanda. Especially how we get to see its first victim and watch it slowly spread out from there, leaving all of its victims with gruesome, fixed grins. Sweet dreams!
The Raveners dear god, the Raveners. The forest that they're found in is a many-day-long journey, during which the sorcerers in the party must maintain a constant mental shieldó if they even fall asleep, the party may be doomed.
Not to mention the Grolims. Torak demands Human Sacrifice, and the Grolims oblige all too enthusiastically. Once, we're treated to the pleasure of a rather graphic description of a slave post-sacrifice.
One per hour per temple. Across the two continents. During some 7000 years.
Plus, the Rivan Codex mentions a yearly special event -for some 3000 years- where exactly 2222 sacrifices (specifically including a newborn child) were offered
Maragor. The combination of its insanity-inducing ghosts and backstory make the mercifully brief time spent in Maragor some of the most horrifying moments in the story.
The backstory is made even worse by the prequel, Belgarath the Sorcerer. Belgarath was rather friendly with the women of Maragor, especially after Poledra's "death", which gives him a certain personal perspective on the genocide.
It should also be noticed that the Tolnedrans only used the Cannibal Tribe aspects of Marag culture as an excuse. Their real motivation was all that free gold lying around, and the fact that the Marags were totally disinterested in giving it to them. That's right, they murdered an entire race for a bit of gold!
The demons. They hail from another universe, they've got the potential to circumvent the prophecies and their attitude towards humans is best summed up by Nahaz's quote about Urvon: "I need this thing."