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Nightmare Fuel: Deltora Quest
Many of the monsters (flying leeches? Lilies that numb you with their pollen, then start eating your flesh? Seriously, Deltora's like the fictional equivalent of Australia when it comes to wildlife that seems tailored to kill humans) and enemies (particularly in Shadowlands).
The Granous, who force their victims to play sadistic games and bite their fingers and toes off.
For added fun, those flesh-eating plants? They're partners with blood-drinking ones. And they're grouped together on the same island. So if by some luck that you're shipwrecked on that island? Watch where you're going and hope you have adequate protection. Even funnier? The author is Australian.
The Sister of the South. ESPECIALLY the guardian: Paff, controlling a monster made of black sludge that suffocates and poisons anyone it comes across. Ugh...
The Grey Guards, whose poisonous projectiles cause agonizing deaths. In Dread Mountain, we meet the being who generates the poison they use: Gellick, a giant toad that can kill by spitting.
The Glus. Especially in the anime where it's capable of healing itself and has an elastic body.
Also, for added Nightmare Fuel, the Glus is one of the two enemies guarding stones that Lief and co couldn't kill, the other being the Hive. The Maze of the Beast is arguably worse, though- a network of caves full of water, occupied by the Glus, which kills anything it comes across. If you stop to rest and lean against the walls, they will solidify around you. The walls are full of the skeletons of people who couldn't get free. When Lief and co went in there, they didn't even try to kill the Glus, they just realised that the only realistic way they could live would be if they found the stone and got out. And then there's what happened when they found Milne...
The Hive is much worse than the Glus. The Glus is stuck in the Maze, doesn't seem to be able to reproduce, and will be very easy to kill if anyone in this universe ever invents gunpowder (and let's hope it's Deltora and not the Shadow Lord who does it first) or indeed if the protagonists hadn't had their weapons taken away before being thrown into the maze. But how do you kill millions of pieces of living sand? The Hive has been expanding for centuries, it seems, and I can't think of any way of stopping it. It's like a slower and much harder to kill version of the Gray Goo unleashed at the end. Especially in the anime.
And then there's the Sand Beasts, mainly when they are first encountered, killing the pursuing Grey Guards. The scene is horrifying in both versions for different reasons, but in both, Lief, Barda and Jasmine can only watch, unable to move for fear of being detected. In the anime, the Guards are devoured whole, and they scream the entire time, the final shot being a lovely one of their arms reaching hopelessly into the air, and then the Sand Beast being shown in the position of gulping them down. Oh yes, the Sand Beasts are also twice as large in the anime as they were in the books.
Don't forget the Hive's hoard of shiny things, piled into a pyramid which is held together by the bones of every living creature that has been lost to the Sands.
Thaegan has some moments in the anime. Most of the time she is enjoyably over the top or funny as hell. However, when she decides to get serious she can be terrifying especially when she's laughing psychotically.
Something like 85% of Isle of the Dead seems like it should also qualify - the book, from hitting the Lighthouse right on to escaping from the Lady Luck inspires a creeping horror. The crew.... And then there's what happened to Doran.
The amethyst dragon went to sleep in a sand dune, which grew by the ton as it slept. Lief and co's passing managed to wake it up, but soon Lief and Barda were trapped on the Lady Luck, and so the dragon spent ten days trying to climb out of the sand dune, weakened by centuries of sleep.
Everything about the Lady Luck: Verity being held hostage, the oars, occupied by the rotting corpses of the sailors Jack tricked into serving, a room full of games designed to put whoever plays them in debtů
The special masks in Shadowgate: They look as though the wearer is part-animal, and if worn for an hour, the wearer can never take their mask off. Ever. Lief wears one for nearly an hour, and Barda and Jasmine rip the skin off his face to get it off.
Kirsten/The Masked One's death: She's bitten and poisoned by dozens if not hundreds of snakes, who use her braid to get out of the pit they're in. It's even outright stated that she's only being poisoned slowly, due to her own magical armour, and that she would take a long time to die.
The Arachs from The Isle Of Illusion. Huge, horrible giant spiders.
The raft-dwelling Aurons' predicament: the Aurons living on the island have made a magic dome that holds in all their magic. OK, but it also has been slowly draining the light from the caverns, to the point that until Lief, Barda and Jasmine came along, they expected to eventually end up living in total darkness. Worse, the Arach like darkness and can swim, so if the light went out, the Aurons would have been fucked.
Dain from the anime is a lot scarier than he was in the book. During his fight with Lief, he mutates into a humanoid monster complete with a Slasher Smile that rivals Dark Lief.
The plague from 'Sister Of The South'.
The scene in Valley of the Lost where the Guardian's pets are revealed to be part of him and the subsequent severing of the cords that connect them to him. The things were ugly enough, but the idea that they grew from the Guardian's boddy is horrifying.