The Great Old Ones (most famously Cthulhu) in the writings of H.P. Lovecraft (and the Mythos they spawned) are the Trope Codifier and, to a certain extent, the Trope Namer; "eldritch" was an adjective much favored by Lovecraft, occurring 23 times in his work. This trope probably wouldn't exist in any significant form today if it weren't for him. Usually divided into three groups (the distinction is mostly created by later writers, but it is present in Lovecraft's own work to some degree):
The Great Old Ones, which are immensely powerful beings made not of flesh and blood but of something that can only be called matter in the most basic sense. They traveled from world to world when stars were right, but now sleep, waiting until the stars are right once more so they may rule again (incidentally, when they wake up, they plunge the world into madness and terror). Cthulhu is one of them, and although human cultists call him a god he is actually just his race's High Priest to the Outer Gods.
Some mythos stories suggest that the Great Old Ones or their spawn were responsible for the creation of mankind, leading to frequent descriptions of creatures of whom the most horrifying thing is that there is something "damnably human" in their appearance.
The Outer Gods (Lovecraft referred to them as the Other Gods), which exist outside our universe and seem to be embodiments of various cosmic principles. They are far more powerful than even the Great Old Ones and seem to be responsible for the creation of our universe (as well as other ones), albeit unwittingly. The most famous ones are the mindless leader Azathoth, the "Blind Idiot God" who resides in the center of all infinity, and Yog-Sothoth, who exists simultaneously in every point in space and time. Their soul and messenger is the Crawling Chaos Nyarlathotep.
The Elder Gods. Lovecraft only used one of these deities (the rest are created by other authors, namely August Derleth, as is the term "Elder Gods"), namely Nodens, Lord of the Abyss. Nodens appeared in a humanoid form (whether this is his true form or one he took in order to not drive mortals insane is unknown) and actually indirectly rescued the protagonist of two stories. Derleth made Nodens the head of a pantheon called the Elder Gods, who were mortal enemies of the Great Old Ones (although some stories seem to place them at the same power level as the Outer Gods). In Derleth's works, the Elder Gods were good and the Great Old Ones evil, which doesn't really fit with Lovecraft's cosmology. Most other writers who have used them make them somewhat benevolent to humans, but only because they want to keep the Great Old Ones asleep, which is also what most humans want to do (what with them destroying the world when they wake up and all).
There are also many species of lesser abominations in the mythos, some independent (like the Flying Polyps) and some subservient (like the Nightgaunts) towards the above God-Things. While not necessarily capable of driving people insane by being observed, Lovecraft has also created a few truly bizarre aliens, like the Elder-Things (not to be confused with the Elder Gods), Mi-Go (described both as crustaceans and fungi, coming to Earth from Pluto but originally from another universe), Shoggoths (literally a Blob Monster capable of changing its form to any shape convenient, once used as servants to the Elder-Things to create their massive empire before also being its downfall), and probably best of all; an alien so bizarre, so incomprehensible, that it can literally only be described as a "colour", and a colour the likes of which has never been seen on Earth at that.
"The Unnamable" is largely a discussion of this trope, featuring a monster-ghost embodying it, but it goes so overboard in its vagueness that it's been suspected of being self-parody. On the surface, it's like a defense of the trope, as it features a sceptical character who thought there could be nothing so beyond ordinary experience ("unnamable") becoming convinced:
"It was everywhere—a gelatin—a slime—yet it had shapes, a thousand shapes of horror beyond all memory. There were eyes—and a blemish. It was the pit—the maelstrom—the ultimate abomination. Carter, it was the unnamable!"
Lovecraft codified the language used to describe these things: "eldritch", "gibbering", "squamousnote scaly", and "rugosenote wrinkly" and such.
The Sepids Dis Acedia, six incredibly dangerous creatures wiping out all life in the Maze every century or so.
The "spirits" in Anne Rice's Queen of the Damned seem to be a cross between this and energy beings. Unlike ghosts who are the souls of the dead lingering on Earth for various reasons, the spirits are said to never have been human at all, they are unable to be seen by people who do not interest them and even those who can communicate with them are often unable to see them. They are described as being enormous enough to fill the sky and one character who witnesses them describes them as being similar to fire, unable to really define its shape in terms the human mind would understand. They are, however, a far more benevolent version of this trope than usual, despite possessing Blue and Orange Morality, except for Amel who created the vampires and even he was more Stupid Evil than anything else.
Inverted in the Blind God of The Acts of Caine. It is as impersonal, awful, powerful, and horrifying as anything from the Lovecraft mythos. The inversion is that it's not really alien. Played straight with the Outer Powers worshipped by the Black Knives in Caine Black Knife.
In the BIONICLE series by Greg Farshtey, there is a character called Tren Krom who was so horrifying it would drive you insane to look at it. It is a crimson blob with hooked tentacles.
The Eidolons from Bitter Seeds are essentially sentient (and malevolent) chunks of the universal substrate.
Rorschach from Peter Watts's book Blindsight is an intelligent, city-sized, incredibly scary vessel. Its "inhabitants", the scramblers, are one of the best examples of Starfish Aliens in fiction. To expand on that: the scramblers are incredibly intelligent (their bodies are about 30% nerve tissue, and it can all be used for processing power) but non-sentient. Even their "retarded children" can think circles around the most intelligent human... but they have no concept of individuality or the "self". Just think about that for a while.
The Mist Wraith, a swamp-dwelling tentacle monster.
The creature encountered in the Wolfsktaag in The Sword of Shannara, which was a composite of machine and monster flesh. It is likely that this is the prototype for the Creeper that appeared in later books.
The Maelmord, a living valley of toxic plant life created by the Ildatch to protect itself.
The Creepers, created by the Shadowen, were, again, creatures of composite machine, insect, and mammal.
The Morgawr. A warlock of disputable origin, humanoid but with scaly skin and shapeshifting properties, apparent immortality, and other powerful magics, including the power to reach inside human skulls and tear out the part of the brain that the soul was anchored to and then eat it, this being how he survived.
The thing living under the town of Prosperous, Maine, and demanding human sacrifice from the townspeople in The Wolf in Winter.
The Swarm of Night of The Chathrand Voyages is what happens when this trope meets Horde of Alien Locusts. An innumerable horde of tiny black insect-like spirits, its purpose is to patrol the borders of the underworld to prevent the dead from troubling the living. When released into the living world, however, it's drawn to massive killings (like mass battles) in progress, finishes them (by killing everyone there who's still alive) and drawing energy from that to increase its size. Once it reaches critical mass, it can- and will- eat a whole planet. The Big Bad released it to do just that, since the destruction of a world was the test his God of Evil patrons demand of any mortal to be judged worthy of elevation to their number. Still, the Swarm is explicitly not evil- just something that should never, ever enter the living world.
In The City and the Stars, Arthur C. Clarke gives us the Mad Mind, an artificially created disembodied intelligence with near-godlike powers, whose creation goes very wrong. So terrifying is it that humans create another one (and do a better job this time) in order to (hopefully) stop it. Humanity is trapped between Scylla and Charybdis on a grand scale: the conflict between the two might destroy the entirety of creation, but implicit in the decision to create the second being is that what the Mad Mind will do if it makes its way back to inhabited space, or remains unchecked for a sufficient length of time, is worse.
The works of Clive Barker frequently include Eldritch Abominations.
The eponymous being from "Rawhead Rex" is an abomination that personifies the male sex drive as a living and malicious organism.
Worshipping and appeasing such a thing is the true purpose of the degenerate, immortal cannibals in The Midnight Meat Train.
The passageway between the worlds in Coraline. At first seeming to be a relatively normal, if strangely unsettling hallway, by the end, it's a wet, furry... thing that's very much alive and incomprehensibly vast and ancient. It makes The Other Mother look trivial, and she's a particularly nasty fairy.
It's also implied that it's far older and more powerful than her, and that, even though she found it and temporarily used it, she has no goddamn idea what it is or how it works.
"The Tower of the Elephant", another Conan story, gives us Yag-kosha and manages the impressive feat of turning one of these into The Woobie, with the utter hell that this creature was put through for centuries at the hands of a particularly sadistic Evil Sorcerer, who he calls a "devil in human form" with very good reason.
In "Black Colossus", said colossus has a truly horrific crush on the princess.
Kim Newman's Dark Future novels feature appearances by Azathoth, Nyarlathotep, and some unpleasant beings from beyond human reality like the Jibbenainosay: a dark spirit which usually manifests as a sort of giant, evil jellyfish.
Throughout the novels, but especially the early ones, a constant danger of the use of magic is that of accidentally opening a rift into the "Dungeon Dimensions", regions with "very little reality", inhabited by nightmarish Lovecraftian monstrosities that crave the reality that those in more solid universes take for granted. They tend to try to invade the Discworld's universe in the vain hope of becoming more real themselves, "with the same effect as the ocean trying to warm itself around a candle." The Dungeon Dimensions don't actually have form; the reason that they look like an incomprehensible mass of tentacled horrors is because when they do acquire some substance by coming close to the real world, they're terrible at it. This does not make them any less terrifying.
At a physical level, in the Dungeon Dimensions the Things are so weak that a small child can destroy them with a sharp push, and drive them away in sheer terror of her when she keeps doing it. This also doesn't make them any less horrifying.
The old dark god Bel-Shamharoth is sometimes presumed to be one of the very few aforementioned creatures that found a way to survive, sort-of, in the real world.
There are other "old dark gods" of the Discworld, too. Ponder is revising for an exam on them at the start of Moving Pictures: he mentions Yob Sodoth and Tshup Aklathep, Infernal Star Toad with A Million Young. Discworld Noir introduced Nylonathotep and makes reference to other dark gods, including Drunken Cthubopalulu.
There's also Dagon (cf. the Cthulhu Mythos), according to The Folklore of Discworld, although it's otherwise only mentioned in the anecdote about someone trying to set up a sushi bar on the site of an old temple on Dagon Street during the solstice. (This did not end well.) Folklore also mentions that the Things may have been some manner of dark gods before they were exiled from existence. In Moving Pictures, it is mentioned that Dagon used to be worshipped in the city that was there before Ankh-Morpork, which covered all the way from present day Ankh-Morpork to Holy Wood; the city is gone, something to do with the Things awakening. It also had something to do with Leshp which sank, and rose, and sank, each time settled by different people. Though there are a lot of pictures of squid and there are slithering sounds and the earliest buildings look like people tried to make sense of geometry they did not fully understand.
The Eldritch Abomination in Reaper Man is not from the Dungeon Dimensions for once, but a result from the Auditors canning the too-amiable Death. They start off as objects that people would collect and forget about, specifically as snowglobes. These snowglobes would "hatch" into useful, traveling objects (shopping carts), that then collect people into the enormous monster-queen-THING that turns out to be a SHOPPING MALL. It's funny, yeah, but also terrifying. Just think of all the shopping malls you've ever been in where you could be anywhere in the world for all the difference it makes, absorbing your time without you realizing it, being herded about like cows. Brr.
Then there's the other Eldritch Abomination in the same book: Azrael, the Great Attractor, the Death of Universes, the being of which all other Deaths are mere reflections, who keeps the Clock, of which all other clocks are mere reflections, which tells time what it is. Death travels to ask a boon from Azrael, and we get treated to a shift in perspective as the hilly landscape Death is standing in suddenly shifts, as the view zooms out, to an open hand.
Pratchett also won't let us forget that "eldritch" and "elven" have the same linguistic rootnote perhaps; some etymologists suspect that it comes from el- "other" + rice "kingdom" instead of from ælf "elf" + rice; in either case it would mean otherworldly.
The Things have not appeared for a long time due to the themes of the series moving on. However, in Thief of Time, we see statues of them in the History Monks' gardens — and the Auditors of Reality are placed alongside them and said to be the deadliest of all. They are inhuman intelligences responsible for maintaining the laws of the universe that appear as adversaries in the novels due to their hatred of the disorder of life and individuality.
The spirit of Holy Wood and the Music could be considered similar entities, although their intentions or methods weren't necessarily malignant.
Played for Laughs in Men at Arms. One footnote lists animals on the Discworld even weirder than gargoyles, among which is the Shadowy Lemma, a creature which exists only in two dimensions and lives on a diet of mathematicians.
The Bugs from Phillip Palmer's Debatable Space. Their name isn't very evocative of what they are, but immediately after discovering them, humanity sacrificed millions of people to put up thousands of indestructible, uncrossable walls between the Bugs and the rest of the universe. Despite this, the leaders of the government live in unending, mortal fear of them. This is all entirely justified. The Fire Beasts are also implied to be remarkably friendly, apathetic versions of this.
Part of the Doctor Who Expanded Universe declared H.P. Lovecraft's creatures canonical and had their names originally bestowed by Rassilon. As if Cthulhu, Hastur (aka Fenric, apparently), and the Fendahl weren't enough, a number of characters (most notably the creators of the Land of Fiction and Compassion, an EU companion who became a TARDIS) have been upgraded to this kind of thing.
The Faction Paradox spin-off expanded on the Yssgaroth, things from another version of history accidentally accessed when the Great Houses set up their version. Inimical to the existing universe, no one's sure if they were multiple creatures or just different aspects of one entity, or even if they were alive at all and not just "symptoms of a timeline that had already started ripping chunks out of its own flesh", and if they took people into their own universe... well, they've been known to string a victim's nervous system out over a planet while keeping it alive and able to sense pain. And it's implied that the Great Houses are secretly studying a way into their universe for military purposes. Just for additional horror, they're connected with the Great Vampires from Doctor Who, which came out of nowhere and swarmed all over the universe, each one capable of sucking a planet dry.
Continuing on Faction Paradox, we have the extradimensional realm known as the Eleven-Day Empire (created by mutilating Earth's history and erasing eleven days that never existed). Moments after its creation, the Faction went there to set up shop. They found the place was already occupied. Which didn't stop them from making a deal with the Very Nice Gentlemen and settling down like they liked. They even got several of their new friends (implied to be the living embodiments of multiversal laws) to defend their new home.
As are their TARDISes. At one point, the disguise that prevents human companions from wanting to pull their eyes out collapses, and we're told one of the controls tries to bite him. At the very least, the Nightmare Child is this. Possibly also the Skaro Degradations, the Could've been King and Horde of Travesties as well
The Outsiders, beings that exist beyond 'The Outer Gates', or the limits of known reality. They can only be summoned by mortal magic and are considered so dangerous that not only is summoning them forbidden under the Laws of Magic, but a member of the Senior Council (The Gatekeeper) has the full time duty of monitoring any possible incursions. These things are so dangerous that just learning about how to summon Outsiders is punishable by death. With good reason. Also, Outsiders eat magic and destroy reality just by being present. It takes wizards hundreds of years to learn how to even hold their own against them in battle. However, Harry Dresden increasingly seems like a living weapon against the Outsiders - while normally a single Outsider can hold down the entire White Council, Harry Dresden has beaten two distinct Outsider champions, known as Walkers. Specifically, He Who Walks Behind in the backstory and He Who Walks Before in Cold Days. It seems to have something to do with Harry being a "Starborn" and the concept that different places and times (such as Halloween) have the ability to change fundamental magic, including granting and removing it, even from immortals. Perhaps Harry himself has an Outsider-pwning mantle in addition to all of his other accumulated positions.
And there's the Skinwalker from Turn Coat, which reduces Harry to a gibbering mess when he sees it with his Sight. It's a demigod and is a walking source of very, very nasty dark power.
In Ghost Story, we finally see Harry's encounter with He Who Walks Behind. It introduces itself by stating its name, which is best described as a paragraph of emotions and sensations relating to contempt and pure alien hatred for mortal life.
"That", a cultured British voice whispered in my ear, "is the closest your mind can come to comprehending my name."
In Cold Days:
Readers get to see quite a bit more about Outsiders. Namely, that there is a unimaginably massive horde of them trying to break into the universe at the Outer Gates. Yes, that's right, the Outer Gates are not metaphorical; they are an actual place at the farthest reaches of the Nevernever, where the Winter Fae constantly fight the Outsiders' incursions and prevent them from destroying reality.
Also from Cold Days, Demonreach is revealed to be the prison for hundreds of these (Abominations, not Outsiders). They're so powerful that six of the aforementioned Skinwalkers are the least dangerous.
During a vampire soiree in Grave Peril, Harry meets a gentleman named Ferrovax, who's really a Dragon in human drag. First thing is that Ferrovax's level of power ranks above even quite a few deities in the Dresdenversenote Word of God says that Dragons like Ferrovax once served God in things like changing whole continents and the seasons, he even knocks Harry around with magic without any effort. He also describes his true form as fitting in with this trope, saying it would break Harry's mind just to see it. Ferrovax remains one of the most powerful and significant beings Harry has ever encountered (and that's saying something), but Ferrovax remains to be a Chekhov's Gunman, despite having a relatively early appearance in the series without being seen or heard of since.
The "Nameless" are entities that the wizards refer to as the dark powers of the Earth, which are the focus of the oldest religion of the Kargad lands in The Tombs of Atuan.
In The Farthest Shore, the main antagonist turns out to be some crazy wizard who tried to achieve immortality — by creating a hole which nearly sucked the entire world inside it.
Xenos, the first book in Dan Abnett's Eisenhorn trilogy has a downplayed example with an alien species called the Saruthi, particularly because of how fundamentally corrupted they were by the power of Chaos. The Saruthi themselves looked like a flat-bodied, mutant crustacean with five irregular limbs, with an oblate head lifted off of the center by a boneless neck, and had such a well-developed sense of touch that they could verge on being considered almost psychic by human standards. But the thing that really set them apart is that they created four-dimensional landscapes that were incredibly disorienting and unnerving to see, let alone be in and to fight a battle in. The simplest way to describe them is that spacetime appears to be incredibly curved, which leads to gunfire going way off mark, architecture and iconography that wouldn't be possible in Euclidean geometry, and time being unstable.
Wind raced as breath or darkness formed a figure. Something large and unfathomable. As deep as the ocean and more vast. A drum as slow as time and as fast as now. Steady, like a pulse, but greater than any pulse or all. Two wings rose and fell. They rose, he saw the past. They fell, he saw his future. And he saw that the shape before him was endless and that its wings made a great circle in heaven. He saw his own death in those wings, and knew that it had already happened, and that it was still to come.
Final Days has the growths. Massive metal flowers that grow out of the seabed and spreads across Earth, bringing earthquakes, violent storms, and strange lights with them. Nobody knows what they are or what their purpose is, or even if they are sentient or not. The only thing that is known is that they will destroy Earth. Though Stone claims they are actually benevolent.
The Final Destination spin-off book Dead Reckoning has the main character Jess enter what appears to be Death's realm in a dream. There she encounters what is presumably Death's true form - the vaguely humanoid Death is gigantic, composed of constantly shifting, crumbling, and regenerating bones from seemingly "every creature that ever lived", and is covered in what could be loosely described as robes made from what appears to be still living flesh that twitches and squirms. From afar, it just looks like a dark mass, and it's constantly emitting a noise that sounds like static and "thousands of birds all taking flight at the same time", while its eyes are completely blank, dark voids. Also, anything in proximity of it ages rapidly.
The Blight, from Vernor Vinge's A Fire Upon the Deep, is a post-Singularity version. It's a five billion years-old god-virus, with no apparent goals except endless expansion. The effect of any form of contact with the Blight proper is nowhere near as merciful as simply driving you mad. Instead, you are instantly turned into the Blight's fleshy terminal, and then are eventually driven mad by becoming a helpless prisoner in your own mind. And it can propagate through computer networks, including interstellar ones. And it can kill the local equivalents of Physical Gods - to which it appears hideously disgusting even before revealing its true nature (mere mortal minds cannot even comprehend the true nature and complexity of the Blight).
Tais Teng's Glass Spears anthology features Nesquaam, the Elemental Darkness. He is that - pure darkness and absolute cold, but sentient and hateful. His presence will break any mind as long as it is exposed to him long enough.
Simon R. Green's Forest Kingdom books contain several types of Eldritch Abominations in addition to the regular evil demons. In Blue Moon Rising, there is a giant worm thing that devoured a mining town; in Down Among The Dead Men, the Big Bad is explicitly named as an evil from beyond the dawn of time; Blood and Honour has a castle slowly turning into one (a room digests its occupants at one stage, and a person is turned into a living doorway to a dimension full of eldritch abominations). Amongst several others.
In the Deathstalker series by the same author, there are multiple occurrences of such abominations.
The massive AI planet Shub exists in more dimensions than humans can perceive and is extremely unnerving for them to look at and capable of causing insanity in some.
The Recreated are entities formed from the disembodied spirits of billions of sentient beings who died when the Darkvoid Device destroyed their planets. Driven insane, they merged with the subconscious mind of the Darkvoid Device and used its power to manifest as civilization-destroying abominations.
The Terror is an entity capable of not just destroying planets, but it actually eats suns to refuel itself; its mere presence is enough to drive entire planets into insanity.
One human esper was so vile and twisted that when she was broken into four individual beings, the Uber-Espers, each manifested as an eldritch abomination.
The Medusa from Les Mondes d'Ewilan is a tentacled entity who took over the Imagination, the dimension from which the magic of this world is drawn. Anyone who entered the Imagination for too long ran the risk of being grabbed by one of its tentacles and being devoured by it. And it just kept growing and growing. It was eventually revealed to be an ancient evil god named Ahmour, whose followers were attempting to bring back into the real world. And they succeeded.
The Other from L'Autre is an ancient entity sealed long ago wishing to take over the world, and divided into three parts, all with a different role : Jalaab (the Strength) serves as a brute force to eliminate threats, Onjü (The Heart) wrecks chaos by causing natural catastrophes and playing with people hearts, and Eqkter (the Soul) enslaves humanity by starting wars and turning men into cowards.
The Spectres of His Dark Materials are semi-corporeal creatures which devour the consciousness or soul of adults (they are invisible to children and have no interest in them). They not only come from the Abyss, but they are a section of it. So they are manifestations of emptiness itself running around.
The House itself. While looking at it isn't immediately maddening, all attempts to understand it or classify it fail, and it moves, reshapes, and exists in manners that should not be possible.
It is possible that The House actively attempts to avoid comprehension by humans and, by doing so, drives everyone vaguely related to it insane.
Alternately, if we take the poem that ends the book into account, yet another explanation emerges: The House is the interior of Yggdrasil, the "tree" that holds the universe, and the Minotaur is, naturally, the demon-serpent Nidhoggr, gnawing at its roots. All things considered, this may not be much better.
The same series also gives us the Vom, which, while more on the world-devouring scale than galaxy, fits several of the requisite criteria: inscrutability (it's a huge, black... mass), exponential power growth, alien thought process (it lives only to devour all life on the worlds it comes across), strange origin (possibly extragalactic), immunity to conventional weapons, and Mind Control/Mind Rape abilities.
The Lions and Tigers and Bears might also count, though they are benevolent towards humans and try to help them as much as they can. The Technocore certainly thinks of them as Eldritch Abominations.
Yog-Sothoth pulls a brief appearance in the Illuminatus! Trilogy, as well as the Lloigors, Tsathoggua, and the Shoggoths; there is also, towards the end the Leviathan, a ridiculously immense, vaguely pyramid-shaped, single-celled, pre-Cambrian monstrosity with a single eye towards the top of each of its four sides as well as far too many tentacles. This being the Illuminatus! Trilogy, that's not the weirdest thing in the novels...
Stephen King is, as we all know, particularly fond of creepy-ass creatures.
In IT, the eponymous monster is perceived as a Giant Spider by the protagonists, because this was the closest analogue that their rational minds could find for Its appearance. Attempting to fight IT can result in one's mind being flung beyond the edge of the universe, then being driven mad by the Deadlights (which IT is merely an appendage of). After the protagonists succeed in killing IT, they magically forget about the entire incident; apparently, this was the only way they could have lived a normal life afterward.
The Turtle in the same work could qualify for this trope. Case in point, the turtle created the whole universe by being sick.
Stephen King and Peter Straub got together to write The Talisman, a horror fantasy novel which is chock full of horrific creatures and mutants, the most disturbing amongst them is probably a mewling tentacle creature that bleeds ichor filled with biting white worms.
The burial ground in Pet Sematary could very well be one of these outright if it isn't possessed by one. Either way, it's pretty safe to say that it's probably more than just mere haunted ground. In the book, the creature is implied to be a wendigo, and while the protagonist is a bit too crazy at the time to be sure, it's possible that he almost ran into it.
The short story "I Am the Doorway" is about a former astronaut who becomes the conduit for an Eldritch Abomination, manifesting in the form of golden eyes on his hands. In an unusual spin on the trope, though, said Abomination isn't malevolent — it's terrified and disgusted by our world, which is as alien to it as it is alien to us, lashing out violently at the horrors it's forced to witness.
In From a Buick 8, the titular car... isn't a car. And things come out of it...
He's implied to be just another form of Randall Flagg, who, as others have said, is more or less just Nyarlathotep with a different name.
And Tak from Desperation and The Regulators; a sadistic, incorporeal monstrosity heavily implied to have no true form, it has no apparent motive other than causing chaos and killing everything it comes across. The effects it has on those it possesses are... disconcerting, to say the least.
As well as another short story, "N", told through the journals of a psychiatrist analysing a patient who believes that by keeping objects "in order" obsessive-compulsive style, he is keeping cosmic horrors at bay (which doesn't seem so strange at first, since that's a pretty common reason why obsessive-compulsives do the things they do). The psychiatrist eventually, following the patient's suicide, takes over his "duty" of keeping things in order and ends up killing himself as well, due to the stress involved in keeping the cosmic horror CTHUN and the rest of its reality out of ours. It's implied that even if more people continue the duty, the barrier keeping CTHUN at bay will stop working anyway.
In The Dark TowerBook Three, Illustrated Edition, a print shown during their trip aboard Blaine shows the part of Roland's world that has yet to even begin to recover from the wars that made it what it is. The bird-things may not reach cosmic-level, but what they indicate about the greater cosmos could snap those old neurons pretty damn fast.
Speaking of which, Randall Flagg is implied to be an Expy for Nyarlathotep...
If that's the case, one could then say that the Crimson King is an Expy for Azathoth. Both are all-powerful, the source of evil, and brain dead.
Another fine King creation: in the short story "Home Delivery", a thing best described as a gigantic ball of crawling worms decides to camp over the South Pole's hole in the ozone layer... and causes a worldwide Zombie Apocalypse just by being there.
The Long Boy from Lisey's Story qualifies as well. That thing... when it eats you, you don't die. You just get eternally digested, and you are conscious.
The evil room from 1408. A big point is made about how it's not even remotely human and never was. It's simply a madness-inducing room housing some sort of extradimensional terror that munches on its occupants.
Revival features a kind of "Special Electricity", which Pastor Jacobs uses to heal the sick, and eventually, reverse death. The Special Electricity is eventually revealed to be the psychic runoff of an ancient being known as Mother, which rules the afterlife, called Null. In Null, all humans who die are herded by enormous ant-monsters to toil and serve Mother and the other Great Old Ones. There is no hint of anything else after death. After the protagonist trespasses in Null, all of the hundreds of people Jacob had healed with the Special Electricity commit murder-suicides, implied to be a punishment from Mother.
To make it a little clearer how bad Revival's example is, an extrusion of Mother followed the protagonist back into our world, and it was made up of the screaming faces of Pastor Jacob's dead wife and son.
The Ancient Enemy from Dean Koontz's Phantoms is a massive, lake-size mass of black sludge, older than the dinosaurs, and consumes other life forms as sustenance, and is able to perfectly mimic any creature it consumes. It can create small "probes" or "phantoms", imitating consumed life forms, to go forth and hunt more prey, obeying the orders of its "hive mind". In addition, the creature absorbs the mental capacity and memories of those it consumes, so its mind grows more powerful, intelligent, and self-aware over time. Besides being able to mimic real animals and people, the creature can also form phantoms based on mental images from its victims; it takes sadistic delight in creating phantoms in the shape of religious demons and monsters to terrorize its victims before killing them. The creature also apparently likes to think of itself as The Devil. It even has human cultists.
The infovore from The Atrocity Archive, the only survivor of a reality where the Nazis actually managed to pull it off. Only thing is, once they unleashed "the frost giant" on the Allies, they quickly began to realize that the thing was draining the heat from... well, everything. Just sixty years later, that universe is nearing entropic heat death, and the infovore wants out...
The Sleeper in the Pyramid on the Dead Plateau in The Fuller Memorandum, a nameless thing kept docile through a wall of crucified, undead victims maintaining a quantum observer effect. Nyarlathotep is also name-dropped - and described as being several orders worse than the Sleeper - and quite a few of his cultists show up (they're not nice people). There's also the Eater of Souls, aka TEAPOT, aka Angleton.
The Feeders in the Night, a very weak form of supernatural nasty that possesses people and turns them into shambling zombies that spread the condition instantly through touch. Those affected are easily recognizable by the glowing, pale green worms writhing inside their eyes.
In an amusing aversion of how the trope usually goes, H.P. Lovecraft is described as a hack writer who cobbled his stories together out of bits and pieces of actual eldritch lore. He did, however, have a run-in with a certain Mother of a Thousand Young in his youth, and came out of it mostly unscathed.
The Eddorians of the Lensman series. They came from another universe (bringing their planet along with them), they have third-stage minds and so can lethally Mind Rape any lesser mind, and the concept of their very existence was so disturbing that it had to be kept secret from both Civilization and Boskone, even though the Eddorians were the founders and ultimate rulers of Boskone.
Thomas Ligotti has created a truly prodigious number of these, most fairly unconventional in presentation and overall manifestation. Perhaps the strangest is the entity central to "Ten Steps to Thin Mountain". Think about the implications of the House of Leaves entry above. Apply this to a meme. Or a stray thought. This is what Thin Mountain is.
First Person Pronoun in The Long Earth. A sentient biosphere that seeks to expand to every parallel Earth, consuming every life form living on those Earths in the process. Her very presence causes migraines in those who are in worlds near the world she is currently in.
China Miéville's Perdido Street Station has the slake-moths - monstrous, insectoid creatures that devour minds. Not literally, what the creatures feed on is the very sentience of their prey itself, leaving their victims utterly mindless shells. How terrible are these abominations? At one point, the government of New Crobuzon attempts to strike a deal with Hell to get them to intervene and stop the threat, and the demons are too frightened to get involved.
And then you have the Weaver, who the New Crobuzon government turns to when the demons turn them down. It's a gigantic spider that exists between dimensions and is capable of traversing them as easily as we would walk down the street. It is also batshit crazy, speaking in the "flight of ideas" style most often seen in unmedicated schizophrenics and capable of doing anything to anyone, friend or foe, merely because it seems "fitting". During the brief time that the heroes are in its presence, the Weaver cuts off the ears of everyone in the room for reasons known only to itself. It also repaired the ears of some of the people, again for reasons unknown. The Weaver encountered in the book has an obsession with scissors and happily accepts them as gifts, if the term 'happy' can be applied to it. Apparently, it enjoys collecting things in general, as it is mentioned that before its obsession with scissors, it collected chess sets.
And then there is the Torque, described by one character as a tumour that aborted itself from the womb that produced the forces of Birth and Death. Whilst not evil per-se, it is a natural force that is almost uncontrollable which warps and mutates matter and biology into horrifying things. Merely trying to research it can turn you into an Eldritch Abomination. It was once used as a weapon; the results of the Torque Bomb were so awful that even after a generous application of Magitek versions of nuclear weapons, there's a country-sized region of the world which isn't going to be inhabitable by anything but abominations ever again. At one point, the protagonist pulls out a book of photos taken at ground zero of the inhabitants (though it may have been implied that there could be survivors) to show to a client. "That? We think it used to be a goat. Or a train."
Oh, and in the middle of a city, there are The Ribs, the partially exposed skeleton of some enormous creature that has been dead for a very, very long time. Attempts to build over it resulted in seemingly structurally sound houses that just fell apart and tools that break long before they should, and attempts to excavate the whole skeleton tended to result in the workers suffering horrifying nightmares or disappearing suspiciously. It was decided that whatever it is is best left buried and uninvestigated.
Then, in The Scar, the second book in the series, there's the avanc, something from another universe that is big enough to pull a floating city, and all that anyone knows about it is that it swims and has at least one thing that could be described as a limb.
The creature from the short story Details is this in spades. It lives in all detailed surfaces, and if you look deeply enough into them, you can see it. The problem is that once it notices you, it will try to get you through all detailed objects, including those in your memories. The only 100% sucessful ways of keeping it away from you are cutting your eyes out or killing yourself, which may be better thanwhat this thing will do to you once it gets you.
The same author's Kraken has cults that revere squid as the descendants of kraken "gods"; one cult has a preserved beak, which causes those bitten by it to develop squid-themed mutations. This is part of the reason that the theft of a giant squid is a giant problem for everyone in the mystical underground.
The Nameless from Merry Gentry. When the fey were exiled from Europe and forced to go to the States, which was the last place in the world that would accept them and allow them to keep their courts intact, they performed a spell which removed a lot of the more exotic and abominative of their powers so that they would be less likely to misbehave, horrify the humans with their other-ness and get kicked out of the last country that would have them. The horrors of their power were gathered into a single... being... that they called the Nameless and was bound by the leaders of the Seelie and Unseelie courts. It is a horror to look upon, and called indescribable by the protaganist when she encounters it.
In The Neverending Story, we have the Nothing. Born from distortions in Fantastica caused by The Childlike Empress' sickness, it's not exactly a creature, but it is as eldritch as anything else. There are no adequate words to describe it. In chapter 1, a messenger tries to explain what happened to a lake that was consumed by the Nothing. It's not that the lake was drained, or that it dried out, because then, there'd be a hole or a dry lakebed there, wouldn't there? It's just... nothing. Later on, Atreyu runs into some wood trolls that came into contact with it. One lost his lower body, one lost the left half of his body, and one had a giant hole eaten away. Only... they're still alive. They just can't... feel anything that was removed. When Atreyu tries to take a look at it from afar, he can't even glance at it straight on, and it pains him to see it even from the side, because his brain simply cannot comprehend the sight of nothing. It is quite simply something that should not exist, and that's because it doesn't.
Whereas the movie version obviously had a hard time depicting the Nothing, a comic bookretelling using Disney characters had it ridiculously easy: they just left the page blank where the Nothing was.
Peter F. Hamilton's The Night's Dawn Trilogy incorporates a positive slew of eldritch abominations: an incident involving a satanic ritual and a passing energy being creates a cross-dimensional link that allows the souls of the dead to come back and possess the living, before secreting entire planets away to their own pocket dimensions. Even worse, the trans-dimensional powers of the possessed, as well as the fact that they have absolutely no idea what they're doing, opened the door to a range of other, semi-scientific eldritch horrors, by far the worst being a dimension of almost infinite entropy which, if intersected with our universe, would suck it dry like a vampire. Things get so hopeless that it a literalDeus ex Machina to sort the whole mess out. Said vampire-dimension, the "Dark Continuum", has, at its center of mass, a... phenomenon called the "Melange". It's made out of the immortal souls of everyone ever trapped in this continuum, all of them writhing in eternal agony at well below zero degrees Kelvin, most of them incapable of accumulating enough energy to break free. Those that do temporarily become vampiric bird-like Orgathe, doomed to wander the empty continuum until they eventually re-join the others in the Melange.
The resident super-civilization of the Night's Dawn universe (so powerful that their empire consists of a collar of planets orbiting the same star) feel horrified and threatened at the prospect of our Universe intersecting the Dark Continuum. That's how bad it gets.
OrannistheDestroyer in the Old Kingdom series. He is alternately described as a sphere of light and a column of fire, and is immortal. His can is his own frozen body, split in half, buried under a hill, and encased by a seal of seven different materials, including bone. He destroyed the world several times before the Seven finally imprisoned him. Yes, that's correct: it took seven GODS to hold this thing. And his only real restriction is his power and movement; he can still communicate with and manipulate those around him. This is the most terrifying, unnatural thing to come from the terrifying, unnatural Kingdom.
The other Bright Shiners may qualify as well. Granted, we only see four out of nine: Orannis, who's a living nuclear explosion, Kibeth, whose true form is a silver Hell Hound, Yrael, a whirling vortex of fire and lightning, and Astarael, a tall woman with power over time, and the river of Death flowing around her.
Chaos and Old Night from Paradise Lost by John Milton. Lucifer passes through their domain on his way to Earth, and as they obviously bear a grudge against God, they allow him safe passage. The weird thing is, we never find out exactly who or what they are, they just are. And, from the look of things, clearly have a history with God!
While the titular children in Miss Peregrine's Home for Peculiar Children possess startling unnatural abilities themselves, they pale next to the hollowgasts, former peculiars that, through some horrifying means, have made themselves immune to the passage of time, feed on the souls of peculiars (though they will eat normals and animals if pressed), and are only visible to a few peculiars as horrible masses of eyes and tentacles. If they eat enough souls, they become wights, which can pass for humans but are only slightly less horrifying.
Legrys Mor in Murder at Colefax Manor, which is described as having the form of "an amorphous mass of undulating, inky blackness" with "thick, slimy tendrils" supporting it above the heart of a whirlpool. Depending on the ending, it can be seen escaping out to sea if the manor is destroyed. Bonus points for actually being referred to as eldritch.
In The Power of Five, the main antagonists are the Old Ones, godlike creatures clearly inspired by Lovecraft that used to rule Earth before the humans defeated them ten thousand years ago and sealed them in another universe. The Nazca Lines were created as the seal, and the animal shapes drawn into the Earth were actually representative of each of the Old Ones, the familiar animals being the closest approximation the human mind could come to the Old Ones' horrifying appearance.
Asmodeus, Mrs Deverill's cat, is implied to be one as well. It not only perfectly understands English, it apparently survives being shot with a shotgun, is implied to have killed Tom Burgess, and follows Matt around. It is also unaccounted for at the end of Raven's Gate...
Whilst they don't necessarily fit this trope per se, the people of Lesser Malling are at least inhuman to some degree. Both Sir Michael and Claire Deverill are noted to be incredibly fast and strong for people of their age. Mrs Deverill also seems to enjoy eating uncooked meat and looks exactly like a portrait of her "ancestor".
The portrait itself acts like a CCTV camera and even directs Matt to go to bed on one occasion.
The Diary of St Joseph of Cordoba has this effect on just about everything around it.
Many of the denizens of Hong Kong in Necropolis.
The Repairman Jack novels of F. Paul Wilson had the Ally and the Otherness (who see the world as a poker chip) and The Lady (who seems to be the personification of Earth's life).
The aliens in Arkady and Boris Strugatsky's Roadside Picnic, by proxy. These unknown aliens only stopped for a flying visit, but the detritus they leave behind, including inexplicable objects and strange phenomena (many of which are lethal) defies explanation. The best analogy given in universe is that it's as if they just stopped for a picnic on their travels, and we're just like the insects in the grass by the roadside. All that's left is their rubbish. Some of the phenomena themselves also count as this.
Skulduggery Pleasant has the Faceless Ones, so named because they cannot be looked upon in their true forms without driving the observer mad and can only manifest by possessing humans, melting all features from their faces in the process. They are the former rulers of this reality, before their slaves, the Ancients (the first mages), managed to find a weapon capable of driving them into another reality. They are described as being so evil and sadistic that even their own shadows were afraid of them. A creature cobbled together from several monster parts including the torso of a Faceless One's host took a small army of mages to kill. When they finally appear, Valkyrie gets only a passing glance at one and is temporarily driven into a catatonic state by its impossible geometry and biology. Skulduggery explains that if they successfully return, they will wipe out half of humanity and then work the other half to death, before destroying the Earth.
Almost worth mentioning that these all-powerful demon gods only drive the nine-part series for the first trilogy, as they soon move on to a sorceress called 'Darquesse', who's indicated to be able to take on the Faceless Ones and win.
Oddly enough, the "angels" from C. S. Lewis's The Space Trilogy show some of these traits. They exist on a profoundly different level than us, have strange geometries, and dealing with them can be terribly unsettling. One of the characters even notes that the fact that they're benevolent makes it worse; no matter how terrible the evil you're facing, there's always the hope that good will swoop in to save you... but what do you do when facing good turns your brain inside-out?
The Oyeresu themselves, on occasion. In explaining what they mean by "manifesting to humans", they use the analogy of the ways a stone can manifest in human perception. The glorious statue is one possible perception, but so is the sensation you have after it's fallen on your head.
It's also worth notable that the Oyeresu, who are just as good as the Un-man is evil, have to try a few times before they figure out acceptable, vaguely humanoid manifestations; their first try is a bad acid trip — eyes, talons, hurtling shapes in a void full of vertigo. This is, presumably, near the hit-in-the-head end of the manifestation spectrum. If it's not, that's even scarier.
The Un-man in Perelandra is a zombie-like human whose evil is so pure and different from that of any other human that it made the protagonist pass out when he first saw the expression on its face. It is controlled by a being who is invisible to us and whose true form is indescribable to humans, not fitting into any of our mental categories. It is strong enough to destroy worlds and yet subtle enough to pass through matter and manipulate human minds. The Un-Man also views intelligence as just another 'tool' and will alternate between an eloquent speaker, to a childish being that enjoys killing small animals and annoying people by saying their name over and over.
The weird angels actually make sense; see the religion/mythology entry.
In the Special Circumstances series, these are one of the many ghouls, goblins, and other supernatural baddies that the title organization is supposed to handle. An Old One is Barbara Everette's introduction to the world of supernatural hostiles in Princess of Wands.
The second part of Queen of Wands involves one of the lesser known Old Ones and its spawned servants. Looking even at just some cellular material left behind on the ground by the passage of one of the servants is shown to induce Sanity Slippage in the observer, and Special Circumstances members have trouble with keeping their wits about them when looking onto it.
The Ro from the Species Imperative trilogy: beings that exist outside of normal space and whose very speech makes one feel as though they are being ripped apart.
There's an arguable case in the Star Wars Expanded Universe novel The Crystal Star, with Waru. "Hethrir's scientists breached the walls between dimensions and brought into existence a massive slab of meat covered with shining golden scales. Though this entity, Waru, lacked discernible sensory organs, it was highly intelligent and could communicate in a deep resonating voice." The scales were variable in size and a syrupy ichor oozed from between them. The ichor could be breathed by humans, and it was Bigger on the Inside. It was promised a way home by the man who summoned it, and it worked with him and healed the sick, was worshipped, and ate people to replenish its healing energy. It was always lonely and ended up eating the guy who summoned it before collapsing in on itself.
While in the Star Wars EU, the newly-introduced Abeloth from the Fate of the Jedi series most definitely qualifies.
The mention of tentacles and the associated imagery does not help...
Her home planet: a place in the Maw where plants eat animals which also happens to be the location of Force purgatory. Useful for something that sustains itself by eating force-sensitive souls.
The 2010 Unknown Regions RPG supplement also added the Mnggal-Mnggal into Star Wars' growing list of EAs. It's a formless black goo that takes over a host and devours them. It wants to consume all worlds in existence, which would be bad enough... but it delights in tormenting sentient beings even more than it does taking them over. Doesn't sound too bad by the standards of alien horrors in Star Wars... until you learn that the reason the Unknown Regions have been cut off from the rest of the galaxy since time immemorial is that the Celestials thought that the Mnggal-Mnggal was too much for them to deal with! Word of God says it and Waru are the same sort of being.
The Three Stigmata of Palmer Eldritch (1965) is a novel by Philip K. Dick. The novel involves the use of mind-altering substances to create a fantasy world, similar to the Sims. Palmer Eldritch is a disturbing character, changing form, but always displaying a robotic right hand, artificial eyes, and steel teeth. Eldritch's true nature and the nature of his business is central to the plot. The theme of Eldritch's dangerous drug marketing is later revisited in Dick's A Scanner Darkly, to which the eponymous film is largely faithful.
Klæl in The Tamuli. Klæl is a sort of counterpart spirit to the one within Bhelliom, which creates worlds; Klæl runs around trying to conquer them, and unleashes armies from other worlds from its wings. It took the combined efforts of the Elder Gods of Styricum, of whom Azash is one, to bind Klæl away previously. Klæl initially appears as a giant monster surrounded by lightning and fire, although it appears Klæl's true form is more like a humanoid shape of pure red light.
We do not speak of 'it', nor do we speak of 'him'. We speak of Klæl.
A minion of the Big Bad, Azash, from The Elenium. An utterly alien creature that comes from someplace else, it's a creature so bizarre and alien that even the Big Bad, an elder god (meaning a god that existed before humans existed, making him an eldritch abomination in his own right), can only control it in its larval form. In its larval form, it's a man sized humanoid... thing with an utterly alien face that mortals can be hypnotized by just by looking at the glow radiating from it, and has powerful magical abilities. Once matured into adulthood, it is utterly uncontrollable, invulnerable to any force, whether man, god, or demon, and it is stated that if ever a male and female were brought into the world at the same time and given the chance to mature, there would be no way to stop them, and they would ultimately turn the entire world into a nest. Thankfully, once matured, their first and foremost desire is to find a mate to the exclusion of all else, and it will search until it starves to death if one is unavailable.
John Hodgman's That Is All features 700 Ancient And Unspeakable Ones (AAOUs for short) that awaken from their slumber in 2012 to usher in the Global Superpocalypse.
Gandalf: Something has crept, or has been driven out of dark waters under the mountains. There are older and fouler things than Orcs in the deep places of the world.
Gandalf: Far, far below the deepest delving of the Dwarves, the world is gnawed by nameless things. Even Sauron knows them not. They are older than he. Now I have walked there, but I will bring no report to darken the light of day.
Then the Unlight of Ungoliant rose up even to the roots of the Trees, and Melkor sprang upon the mound... and their sap poured forth as it were their blood, and was spilled upon the ground. But Ungoliant sucked it up, and going then from Tree to Tree she set her black beak to their wounds, till they were drained; and the poison of Death that was in her went into their tissues and withered them, root, branch, and leaf; and they died. And still she thirsted, and going to the Wells of Varda she drank them dry; but Ungoliant belched forth black vapours as she drank, and swelled to a shape so vast and hideous that Melkor was afraid.
Ungoliant later attempts to eat Melkor. She would have succeeded, too, if the Balrogs hadn't pulled a Big Damn Heroes...Yes, really. This is Melkor, Boss of Sauron, Strongest of the Valar (powers of the world/god-like beings/Angels), and only subservient to Eru (the creator/Super-God/The God) himself. Ungoliant was one big bad mama.
She is said to have "descended from the Outer Darkness, maybe, that lies in Eä beyond the walls of the World.note Outer space, for those of the scientific nature." After breeding with other... things... note Giant Insects? Maia? Monsters created by Morgoth? in a valley so corrupted by her presence that, 400 years later, Beren is the only mortal to have passed through it and survived, she just... wandered off. "Some have said that she ended long ago, when in her uttermost famine she devoured herself at last." But they Never Found the Body.
Her daughter, Shelob, may count as a minor abomination.
Some version of the voyages of Eärendil state that he found her in some southern sea and managed to kill her. That certainly outdoes his slaying of Ancalagon the Black, greatest of the winged dragons.
It's sort of implied that any fallen Ainu will take on eldritch characteristics. Morgoth did note "... greater than a mountain with its head above the clouds, crowned with smoke and fire, and the light of his eyes drove the lesser Ainur to madness." and Sauron has a fair few (such as becoming a 'shapeless, dormant evil' after the destruction of his body, causing a 'shadow of fear' to fall by beginning to manifest himself again, and his spirit towering over Mordor like a black cloud after his final downfall. Even after his downfall, he becomes, as Gandalf puts it, a 'spirit of malice that gnaws itself in the shadows'). Even Saruman may have gone this way given enough time, as there is a recurring theme of him being a kind of lesser Sauron as Sauron is a lesser Morgoth. But these guys are more like your garden variety devils than true Eldritch Abominations.
Saruman also turns into a towering, cloud-like form when Grima kills his physical form in the book, reaching into the West, before being blown apart. By a wind nobody can feel.
And then there's this disturbingly plausible theory that Tom Bombadil of all things may in reality be something very, very sinister.
A. Lee Martinez's Too Many Curses gleefully wallows in this trope, giving us The Thing Which Devours, The Beast Which Annoys, The Black Plook, The Monster That Should Not Be, The Hideous Impaler, and The Door At The End Of The Hall.
The Braided Path features this with Aracarat. The Edgefathers and Weavers count as well, being its vessels.
The Inheritance Cycle has Shruikan, Galbatorix's dragon, who became twisted through dark magic and grew to immense proportions and power, far larger and more powerful than anything has a right to be. When he flies overhead before the final battle, he blots out a large portion of the sky. And he's an Omnicidal Maniac and a Woobie, Destroyer of Worlds.
In A Song of Ice and Fire The Others are teriffying creatures made of ice, who raise the dead. George R. R. Martin describes them as "a different sort of life... inhuman, elegant, dangerous."
In addition, folklore claims that their world is found in the eye of an impossibly large giant known as Macumber.
Fritz Leiber's short story "A Bit of the Dark World" involves and entity that can literally turn light into darkness and make an entire hillside vanish without a trace. It (she?) is described thus:
"There reared out of the canyon, facing Franz, towering above him, a filiment-trailing form of shimmering velvet black...that looked like a giant hooded cobra, or a hooded madonna, or a vast centipede, or a giant cloaked figure of the cat-headed goddess Bast, or all or none of these."
Shub-Internet in Mr Blank is the personification of the actual internet... and it's become a Cthulhoid deity that eats porn.
Eldritch abominations are mentioned in passing in the sequel as being yet another thing our retired hero doesn't want to deal with.
In The Man With The Terrible Eyes, there are the Void monsters stored in Iotech's basement, the beetles under the couch, the dog, and the Man with the Terrible Eyes himself.
The Grievers from The Maze Runner are small versions. Half mechanical, half organic killing machines. All mysteriously and frighteningly lethal. They represent the reality-bending phenomena surrounding the maze; their patterns are unknowable and their presence constant. Meeting one will result in the danger of painful death.
The Crawler and perhaps all of Area X in Jeff Vandermeer's Annihilation is never called this, but the description certainly, disturbingly fits. An inverted tower is found with a staircase winding deeper and deeper into the earth, with an unsettling endless sentence written on the wall by a ceaseless thing. The nameless protagonist descends the stairs and finds it, and the description goes on for confused pages.
The light surged out, blinding and bleeding and thick and layered and absorbing. It so overwhelmed my ability to comprehend shapes within it that I forced myself to switch from sight, to focus at first on reports from other senses. [...] As I adjusted to the light, the Crawler kept changing at a lightning pace, as if to mock my ability to comprehend it. It was a figure within a series of refracted panes of glass. It was a series of layers in the shape of an archway. It was a great sluglike monster ringed by satellites of even odder creatures. It was a glistening star. My eyes kept glancing off it as if an optic nerve was not enough. [...] What can you do when your five sense are not enough? Because I still couldn't see it here, and that's what scared me the most. Why couldn't I see it?
Then she finds it and things get so much worse. As a bonus, a sample tube scraped off its surface reveals that it's made of human brain cells, there is a human being inside surrounded by "indescribable things I could think of only as his jailors", and characters infected by contact with it start "changing" in at first subtle ways, like glowing and gaining Super Senses, with the implication that they might turn into that, or animals, or moss. Oh, and when the Crawler reaches the bottom there will be a "convulsive season of barricades and blood."
William Gibson has one in his short story Hinterlands only called The Fear of the Big Night which comes back when people brave enough to go on interstellar trips via an unexplained jumppoint arrive back to base. All of the ones who took the trip either committed suicide or died shortly after arrival. But hey, they may bring back invaluable objects from elsewhere after a few years absence, so the jumps continue.
In Please Don't Tell My Parents I'm a Supervillain, the team manages to stumble onto one of these completely by accident. After choosing a random middle school to attack and a random semi-rare material to steal, Penny ends up with a jade Artifact of Doom in her lap, and then out comes the dragon. When it's close to defeat, it morphs into this humongous tentacled mass with eyes peeking in from alternate dimensions and such. It's later explained that it wasn't a real dragon, and was just masquerading as one.
In Tamora Pierce's Tortall Universe there is one present in the Chamber of the Ordeal that is used as a final test before knighthood. Not only does the Chamber create a person's worst nightmare around them, any dark character flaw will mean death, or ongoing torture OUTSIDE the chamber should you manage to escape it. It's foolproof, but no one understands how it operates. Word of God has come out to say it is an older god than their current pantheon.
An older lesser god at that, which makes one wonder what the great gods were like.
The creatures in Bird Box are so unfathomable that simply looking at them causes one to go insane.
The Children of Old Leech, in Laird Barron's novel The Croning, are a form of intergalactic parasite that have been infiltrating humankind for centuries, using human bodies to disguise their their true forms. While their motives remain somewhat mysterious, they're clearly not here to make friends, and characters unlucky enough to discover their existence usually wish they hadn't. They can also be found in many of Barron's short stories, including "The Men From Porlock" and "The Broadsword."
The Paradox Trilogy features phantoms, invisible and nearly indestructible beings of pure energy. Their mere presence causes distressing effects on space and time, and they are believed to come from a different dimension.
The star/planet/god Ahriman and the manifold monsters it sends to earth in the Ahriman Trilogy.
The Big Bad from The Behemoth is one and Roger, the lead character, is slowly turning into one. Roger undergoes several Emergency Transformation events where his blood streams from every orifice on his body, turning into coagulated armor and a set of ram's horns, while completely filling in his mouth. The Giant, the villain, is described as a distorted, person-shaped hole in the world, through which violet eyes and yellow teeth float, occasionally coalescing into something face-like. It speaks in unpunctuated ALL CAPITAL TEXT
The being below the water in DO NOT TAKE THE SHELLS is so horribly alien the protagonist can't really describe it in a coherent way. Looking at it definitely has a damaging effect on his mind.
First and foremost, the Crawler. The Biologist is completely incapable of describing it in any concrete detail, and it seemingly shifts from one form to the next from moment to moment, with heavy implications of You Cannot Grasp the True Form. The only detail that remains constant is the single arm that it uses to "write" words on the walls of the topographical anomaly. In the final book, Ghost Bird is able to give a description of the Crawler that is more concrete but no less bizarre or nonsensical.
The "leviathan" that Control, Grace, and Ghost Bird encounter in Acceptance, i.e. what's left of the Biologist after the brightness has its way with her. It's an enormous amphibious creature the size of a building that seems to phase in and out of reality and is covered in eyes, most human, some not. It also communicates via a bizarre and disturbing variety of touch-based Telepathy and is implied to be able to travel between dimensions.
The Dark One from The Wheel of Time series. It's nothing more or less than sentient idea of evil, and all the evil is born from it. It is possible to destroy it, and if someone succeeds, all evil would be destroyed... along with free will, rendering all goodness hollow. Rand sees it as a void - he can't comprehend it.
The Golgotha Series has the Darkling, which existed in the darkness before the Universe's creation. Not even the Creator can kill it, because it predates the creation of death. One of the names given to it is Nyarlathotep, in reference to the Cthulhu Mythos.
The titular Xeelee from the Xeelee Sequence are essentially Sufficiently Advanced Aliens taken to their logical conclusion. They can travel through time, and have used it to improve their technology and power by going back as many as 13.5 billion years. They also use black holes as habitats and construction devices, as well as computers. Their Nightfighters are built of pure spacetime, and can have wingspan of miles. They built a Ring so large that it changed the movement of galaxies, that can serve as a wormhole to another universe.
Their main enemy, the Photino birds, are bizarre creatures made of dark matter. They are shown to be unaware of all baryonic life. Their goal is to turn stars into white dwarfs, in order to prevent supernovas and survive. This makes them the enemy of all baryonic life, including the Xeelee.
From Manifold: Time, The Downstreamers, the far-future descendants of humanity, are essentially gods. They lack any physical form. They can influence events in the past and have more energy at their disposal than entire galaxies, can travel to other universes, create other universes, and their plan is essentially the gestation of a googolplex more, by sacrificing themselves and their history. They also have a Bizzare sense of morality. The only thing they share with modern humanity is a desire to survive.
The Red Bull from "The Last Unicorn". The protagonists wonder what its true form is when alone; its body extends out beyond the walls of a tunnel it's in; it seemingly can't be killed or beaten; and its might is endless.
John Dies at the End and its sequel, This Book Is Full Of Spiders, has the existence of these as a major, ongoing plot point. Multiple eldritch entities (most notably the Shadow Men and Korrok) are constantly trying to find ways to enter and enslave our dimension, each one of them acting independently of each other. It's implied the only reason none of them have succeeded is that their elaborate machinations to do so keep inadvertently sabotaging each other.
Wayne Barlowe's Inferno has the Abyssals, the native inhabitants of hell... yes, demons were not the first living there, they were cast there and hell already had a natural fauna.
From Heaven's Door: The thing dwelling in Heaven, appropriately dubbed "God," is an incomprehensible thing that wants to get into Earth for unknown reasons. What it is exactly is unknown, but it's described as little more than an "existence," incredibly different from normal existence, and as an "eraser mark in the book of space and time." Its presence in Earth alone will be enough to destroy all humanity and twist the world into something similar to its own plane.
The Basilisk counts as one; a giant killer snake that has lived underneath Hogwarts for hundreds of years and the personal pet of Salazar Slytherin who put it there so it could be released by his descendant to kill any wizard children of Muggles aka Mudbloods. The Basilisk can kill anyone who looks directly into his eyes and seeing the reflection of it's eyes will cause petrifaction and it's crazy miracle nobody besides Mony Myrtle was killed by it. The poison of the Basilisk only has one cure: Phoenix tears and Harry would've died if Fawkes hadn't cried on his arm when the fang pierced it, Basilisk fangs are also are strong enough to destroy Voldermort's Horcruxes. Harry was able to kill the Basilisk with the sword of Godric Gryffindor, which absorbs it's venom as well.
The Dementors the guards of Azkaban; ghastly and unnatural even by the standards of the wizarding world, they feed off of joy and can suck the soul right out of your body plus no explanation as to exactly what the hell they are. Even Muggles can feel their presence, though they can't see them, and merely being in their presence can fill humans with despair as all the happiness is leeched out of their bodies from these monsters. They can even affect the weather as Cornelius Fudge explains the vast fog that being covering London is because they're breeding, Harry even muses that the reason why his Godfather Sirus Black (escaped convict) is hiding out in the tropics is because Dementors wouldn't last long in sunny areas.
Thanks to Fantastic Beasts and Where to Find them there's a new unfathomable creature; The Obscurus that are even more horrifying than Demetors despite having a clear explanation to what they are. The "Obscurial" are young wizards and witches who due to being forced to suppress their magic have it explode out of them in a giant dark mass of parasitical magical force. The screenplay of the movie describes the Obscurus as also having white eyes and emits howls and snarls, the Obscurial in question Credance Barebone is even able to control his power at will and rips apart most of New York it took an army of Wizards to destroy him. The most disturbing part comes the fact that Obscurial are just children that have experienced physical or psychological abuse due to being shamed by their magic, Obscurial have apparently been around since ancient times when Muggles where hunting Wizards and Witches down.
Gellert Grindelwald went to New York to try and control Credance and use his destructive power for his own tyrannical ends, there's even the suggestion that Dumbledore's sister Ariana Dumbledore is an Obscurus which explains her condition.
In the movie there's visual similarities between the Obscurus and the Dementors and the idea of the two magical Abominations being related is extra horrifying, and would possibly explain what the Dementors are.
The Shadowthurges in Lockhartand Teague:The Empty Chest worship the Neverborn King, a God-like being that seems to resemble cthulhu if he were covered in parasites and tumors.
The titular Engines in The Engines of Dawn are a race of slug-like, sapient creatures that can naturally enter Hyperspace, taking anything nearby. Like a spaceship. They feed on the molecules they sublimate off their passengers. Their larvae don't know how to control this (and their elders get periodic fits of insane gluttony), so they sometimes eat everything all in one go; including themselves. It's theorized that they were the first creatures in the universe. They're not enslaved by the way; their middlemen are appeasing them by giving them newer and tastier things to consume.
The Zombie Knight has Feldeaths. They're what happens when a huge number of dead souls left unferried for many years gradually merge into one vast, immensely powerful being filled with rage and pain. They attack anyone who enters their teritory, and hunt anyone who retaliates (as opposed to just fleeing) to the ends of the earth.
Harry Moony (or whatever its real name is, but that's how an old folksong calls it) from Ramsey Campbell's The hungry moon. It apparently descended on the moon at the dawn of time, and during the early ages of the Earth frequently used moonlight for coming to Earth and gorging itself on the creatures living there. Eventually druids learned how to summon it and how to satisfy its fits of hunger, before collaborating with the Romans for striking it while it was possessing one of them and banish it to an abandonned lead mine. Naturally, it didn't kill it. Even when being banished, it could influence some sensitive people with the help of moonlight, feed on the hapless animals which would fall into the mine, and even twisting them to become its agents. Once it managed to possess another man who fell into the mine, it quickly unleashed its power and locked the whole valley surrounding the mine into a neverending night and allowed its servants to spread chaos. Fortunately, Harry Moony is hurt by sunlight, but as said above he can block it entirely, and it was planning to expand its influence. He's not even defeated at the end, merely weakened down and sent back to the mine, so it's entirely possible that the events of the book will repeat themselves one day.
The Wandering Inn:Skinner, oh so much. Where to begin? This gigantic bundle of concentrated Nightmare Fuel is an undead Flesh Golem thing that steals people's skin, ripping it from their living body, and integrates it as part of its gigantic self, with the victim's frozen face still visible on Skinner's patchwork "skin". He achieves this by throwing an outstretched arm from inside his mouth. And his eyes (if you can call them that) glow red and give off an unshakeable aura of fear. …Gasp.
The Unexplored Summon://Blood-Sign: The Black Maw that Swallows All, a tornado-shaped mass of black slime, containing a mouth lined with teeth. If a summoner breaks certain taboos, their Material (summoned being) will transform into the Black Maw. Unlike all other Materials, the Black Maw is entirely uncontrollable by its summoner. Additionally, it is capable of bypassing the protective circle (a normally-inviolable shield that protects the summoner) to devour them whole. It turns out that the Black Maw is in fact another facet of the White Queen, making her an example of this as well. The White Queen is another uncontrollable Material (though most summoners are unaware of this fact) and is by far the most powerful entity in the setting. She's so powerful that she was able to defeat an army of Unexplored-Class Materials, who represent the laws of the universe and exist specifically to counter the Queen. Because of this, the Unexplored-Class all submitted to her, effectively giving the White Queen authority over the entire universe.
In The Seichi Chronicles, the Asager Shells are terrifying beats that exist to kill everyone and can appear in any form. They know nothing except destruction and violence. Even the wyvern Asager Shell's scream is enough to be cemented in the main character's mind for decades to come.