The Hive. 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.
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, a gruesome display of how many lives it has claimed.
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.
Gellick, a talking, sapient toad who appears in Dread Mountain, having enslaved the Dread Gnomes. Unlike the Wennbar, Gellick regularly kills his followers just out of spite. It's a massive, bloated creature that oozes a poison that kills in seconds, can spit the same poison, and which demands ever increasing tribute of bugs and vermin for it to eat. If its meals are delayed even the slightest, it throws a tantrum that usually kills at least one gnome. To say nothing of the fact it's big enough to eat gnomes, or the Domestic Abuse overtones of its relationship with its slaves.
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 Guardian of the Diamond is one of the most unnerving villains in the first series. Enslaving countless people as cold beings of un-life yearning for any warmth and sensation, made worse by the fact that they are actually the population of Tora, banished from their home and backed up by his four brutish, bloated canine monsters; Envy, Wrath, Pride and Greed.
The scene where the Guardian's pets are revealed to be part of him and the subsequent severing of the cords that connect them to him is very disturbing. The things were ugly enough, but the idea that they grew from the Guardian's body is horrifying. This horror is compounded when they turn on their master and attempt to maul him to death.
The Arachs from The Isle Of Illusion. Huge, horrible giant spiders that could walk on water with frightening speed using their webs and could tear apart boats and people with ease.
The Masked One, guardian of the Sister of the North, is a disturbing foe. Clad in a dark cloak and bright green mask, it makes a habit of appearing literally out of nowhere many times during the book and kills people each time with growing, nail-less fingers that burn on contact
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 guardian of the Sister of the South is one of the most cunning, powerful and deadly foes in the series. A quadrupedal beast made of black oil with two heads, a dog and a eagle, which make it impossible to surprise attack. It's main method of attack; sending hook-tipped tendrils of oil to rend and tear at anyone who it encounters.
It's other method of killing is even more horrific. Plugging a persons mouth and nose with it's liquid body to suffocate them with relative ease, impossible to stop once it begins unless the monster is distracted or decides to spare you, which is highly unlikely. It kills multiple guards this way, would have killed the Topaz Dragon like this had Lief not warned him, and nearly kills Jasmine and Gla Thon using this method.
It's creation and spreading of the Toran Plague as well as the manipulation of the entirety of Del is the single biggest display of cunning and deception seen by one of the Shadow Lord's servants.
The reveal that innocent Paff is the true identity of this monster, controlling the beast made of black sludge using sorcery that suffocates and poisons anyone it comes across is more than a gut punch to some readers. Ugh...
Deltora is practically a fantastical expy of Australia when it comes to the sheer variety of deadly animals and even plants that show up in the series. Perhaps it's not so surprising, when you remember that the author is Australian.
How bad is it? Part of the reasons that the seven tribes of dragons count as the Big Good of the setting is because they feed predominantly on the various monsters; the present setting is such a Death World because the monsters have had about a century without any predators keeping them in check.
The Granous, a race of Always Chaotic Evilbeastmen who force their victims to play sadistic games and bite their fingers and toes off, one by one, for each time they lose. Once the last set of digits is gone, they eat their victims..
Gets even scarier with Secrets of Deltora, which reveals Gellick isn't some Shadow Lord created abomination, as was originally suspected. It's part of an entire race of such poisonous amphibians called Ooze Toads; the death of the emerald dragons has simply allowed one to grow to such size, without ending up as dragon-food.
The Silence Spider would make anyone arachnophobic. It's not a giant spider... it's just a hyper-aggressive jumping spider with venom so lethal there's no antidote (victims die before any can be administered), which instinctively leaps out at the face of anyone who gets too close to its web. And did we mention their webs are huge and it likes to stretch them across paths through the forest?
Sunrays are Man Eating Plants that camouflage themselves by laying their trapping leaves flat on the ground, looking like a pile of berries in a patch of sunlight. When anything gets too close... snap!
Flesh pythons; not big enough to eat humans, but imagine being in the Forests of Silence at night, when swarms of thousands of huge, flayed-looking snakes come spilling out of the undergrowth...
The Wenn are creepy-looking creatures to begin with, but they also have the charming habit of taking victims as Human Sacrifice for a monstrous reptile, the Wennbar, that they worship as a god.
You can't trust the fruit trees in the End Woods; groves of trees whose fruit-flesh has a soporific effect are the hunting grounds for huge birds big enough to eat humans known as Orchard Keepers.
Pinwheel vipers really fill the Paranoia Fuel quota; they're highly venomous snakes that like to camouflage themselves like chameleons, making it very easy for them to get stepped on.
Imagine being swarmed by land-dwelling sea cucumbers that proceed to smother you with sheer weight of numbers and expelled froth. Then imagine them dissolving your corpse to feed on your liquified remains. Congratulations, you've just envisioned bubblers.
As if sunrays weren't bad enough, grippers disguise themselves as harmless weeds. If you stick a hand or a foot there, it opens a central mouth ringed with hundreds of downward pointing teeth; the more you struggle, the more they rip your flesh apart. And their fangs inject a venom that enhances blood flow. You have maybe a few minutes before you bleed to death and are slowly dragged in to be devoured.
Blood lilies aren't scary... but their pollen has numbing effects, and they coexist in a symbiotic relationship with vermin called "fleshbanes". So you won't even realise you are being Eaten Alive by the bugs until it's too late to do anything.
It's not just the Shadowlord and his underlings who have this. A display of this shows up that's on the heroes' side, in the form of the Dreaming Spring. If the pool's water senses that a drinker is evil, then that drinker is instantly transformed into a tree. Forever.
The entirety of the Forests of Silence, where trees are alive and, potentially, hate all humans.
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.
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 warmth and can Walk on Water, so if the light went out, the Aurons would have been in serious trouble.
The Grey Guards, warrior servants of the Shadow Lord whose poisonous projectiles cause agonizing deaths.
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.