This is based on opinion. Please don't list it on a work's trope example list.
Nightmare Fuel / Shannara
In Elfstones of Shannara, Terry Brooks entered into the realm of horror with the Reaper. Any scene in which this silent, massive, cloaked shadow-demon appeared was terrifying, especially its inaugural appearance in pursuing the heroes at Drey Wood, and Wil's later discovery of what it had done to the Rovers at Whistle Ridge. But the Crowning Moment of Scary had to be the chapter detailing the party's flight through the Pykon. Old, abandoned ruin? Check. Howling, moaning winds? Check. Terrifying discovery of dead bodies? Check. Our hero has a nightmare, only to wake up from it to find himself in a real one? Check. Headlong flight through the darkness of the fortress, knowing the Reaper is behind them, even though they can't see anything, and will eventually catch them? Major check.
Another Shannara series, The Voyage of the Jerle Shannara, features Truls Rohk, a caustic and sarcastic half-shapeshifter, but a close friend of the protagonist Bek. Shapeshifters in this setting are spirits, less individuals than collective minds with multiple manifestations, changing freely as they wish. Truls is almost never seen without his full cloak, and eventually the reader learns why: his body is a malformed, ever-shifting maelstrom of flesh and blood in vague human shape, bits of it mutating, healing, decaying and tearing apart forever with exposed bones and rivulets of blood and half-formed organs. He is... understandably bitter, since he is apparently so hideous his own father wanted him dead the moment he saw him.
Also from Voyage: The Morgawr. A giant lizard man who gains his powers by feeding on the Life Energy of his victims. He turns people into puppets by pulling out their brains and eating them, leaving them with their skills but no free will. He's devoured his way through thousands of victims, delights in feasting on people's minds just for fun, and broke the Ilse Witch as a child just so he could deny Walker her power. The scenes where he feeds on the airship crews are purely this. Antrax's wronks are just as bad: Hollywood Cyborgs slaved to Antrax's drivers who can only obey its orders, even while their still living minds scream for them not to. The fate of magic users is worse: Antrax drugs them into believing they are under attack, and when they try to defend themselves, it drains of their powers, using them as batteries for the rest of their lives.
Then there's the Jachyras, which were so bad that they were turned into Sealed Evil in a Can by both the good and evil faeries long before they started trying to do that to each other. An 8-10 ft tall monster with flame red skin almost hot enough to burn, a barking call that sounds like maniacal laughter, poisonous claws that not only poison your body but your spirit as well, and they literally feed off pain, both that which they inflict, and which they receive. And to top it all off one of them was able to Kill the most powerful druid ever
One often forgotten about is the Caves of Night, which are Exactly What It Says on the Tin—nothing can penetrate the darkness except the Fire Wake and, later, Jair's use of the vision crystal. Worse still is that the caves are full of Procks, some sort of halfway-sentient holes in the floor that endlessly open and close, grinding and chewing anything which falls into them. Naturally this happens to Stythys (himself something of a form of Nightmare Fuel), but the idea of even the disturbing, Foe Yay-indulging Mwellret suffering this fate is still rather terrifying. And while Jair's party does eventually escape thanks to his wishsong and the vision crystal, there's the moment after Stythys's death, when the Fire Wake dissolved and they were left alone in utter darkness, unable to move, with the sounds of the grinding Procks all around them...