The closest you will ever come to actually seeing it.
"You approach the door in the old, deserted house, and you hear something scratching at it. The audience holds its breath along with the protagonist as she/he (more often she) approaches that door. The protagonist throws it open, and there is a ten-foot-tall bug. The audience screams, but this particular scream has an oddly relieved sound to it. 'A bug ten feet tall is pretty horrible,' the audience thinks, 'but I can deal with a ten-foot-tall bug. I was afraid it might be a hundred feet tall.'"
A villain-specific type of He Who Must Not Be Seen, Ultimate Evil is evil so horrifying it cannot be shown on screen. Used when nothing the art department could come up with could possibly be horrifying enough. Or because you have no budget for effects, and need an easy out (see Shaky P.O.V. Cam).
In some cases the Ultimate Evil is eventually shown on screen, perhaps because the heroes are finally at the end of the Sorting Algorithm of Evil and need something tangible to oppose. These cases usually end in disappointment, and prove the original decision not to show anything correct. If said disappointment is intentional on the authors' part, then the villain is just The Man Behind the Curtain.
Sister trope of Nothing Is Scarier, where an entire story's terror factor relies on the invisibility of...whatever it is. Compare Satan. And, for that matter, God, who often gets portrayed this way for entirely different reasons (we hope). Odds are, if multiple gods are present in the setting (or not, even), this character is the God of Evil. Do not confuse with the Bigger Bad, who is literally the most powerful evil force in a given setting. The Bigger Bad may well be Ultimate Evil in this sense as well, but it doesn't have to be, and less powerful evil things can get this treatment too. You Cannot Grasp the True Form is sometimes used in conjunction with this trope; even those who attempt to look at the Ultimate Evil are unable to do so.
Compare Eldritch Abominations, which are just as freakishly powerful and mind-shattering, but more often callously indifferent than actively malignant, though there is some overlap. Not to be confused with Complete Monster.
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Mao-chan parodies the trope with an alien that is too cute to be shown, as it causes all who gazed at it to swoon with heart shapes in place of their eyes. While the program never reveals the alien to the audience as per the trope, the shadow of the creature suggests an amoeba-like form with eye-stalks and possibly small tentacles as well.
Baccano!. While Ronnie isn't technically a "demon", much less pure evil, supposedly his true form is such that humans can't even grasp it. Apparently, the only thing that registers in people upon seeing his true form is an utterly overwhelming feeling of fear. In series, this form is portrayed as just a fluttering shadow on a wall accompanied by a creepy, echoing voice.
Sebastian's true form in Black Butler. Although it has a physical manifestation, Sebastian tells Ciel to close his eyes, and the camera only gives us glimpses. There are black feathers. A lot of black feathers.
Sedna in Umi Monogatari has no real form, only seen as a cloud of red sparkles.
In Fairy Tail, Zeref is played up as this for 200 chapters, with characters commenting on what a horrible killer he was whenever he's mentioned, several demons he created nearly killing people or being part of a character's tragic past, powerful wizards reacting to his name as though he were Voldemort, and evil cults forming dedicated to his worship. When he finallymakes his debut...
Death Note has the Shinigami King, the master of the creatures that feed on human lives. Word of God said that his image is too evil for human eyes. He was finally shown◊ in a bonus chapter.
Diavolo in Jojos Bizarre Adventure tries to invoke this on himself by erasing all traces of his existence, including killing anyone and everyone who's so much as even seen his face including trying to kill his own daughter.
Depending on the Writer, Doctor Doom's face is either apparently one of the most horrible visages in the Marvel universe or has scars that are either entirely absent or extremely small, exaggerated by Doom's ego as life-endingly horrific. We never see under his mask.
Current consensus is that both versions are true—originally it was a small scar, but the armour he has made to hide it was put on too early, before it was cooled, so he really does have horrifying burns all over his body. Also the scar was not caused by a laboratory explosion but by the demon Mephisto scratching his face (the result of said experiment, followed by the explosion) and thus it wasn't simple vanity that drove him to do that; he rushed to put the armour on because he could still feel the demon attacking his face. You could say that Doom's true injuries (or lack thereof) really come from the blows of the comic writers Armed with Canon.
Count Otto von Doom's is shown for one panel in one of the Marvel 1602 spinoffs. It's...not particularly pretty. Especially dark because Count Otto's sobriquet in the original series is "the Handsome".
In El Eternauta, the true invaders are never to be seen, relying on several enslaved races to carry out their bidding. Essentially, all we get on them is that they are the "cosmic hatred".
The Transformers has Liege Maximo, one of the thirteen original Transformers, who is the ultimate counterforce of good. Megatron and the rest of the Decepticons are descended from him.
The same goes for the demon of Paranormal Activity, in which only footprints and the shadow of the demon are visible (3-toed footprints to make clear it isn't human).
Many of the tenets of this trope evolved from the 1942 horror classic Cat People. In that case, the film's budget was very low and the only special effects the production could afford was tatty off-the-rack "man in a cat suit" suits; the director thought it would be much scarier to not show the creatures at all but merely use cinematographic tricks and the actors' performances to suggest them. The effect worked, and has been endlessly copied ever since.
The origin of this particular usage was dramatized in the fictional film The Bad and the Beautiful, in which Kirk Douglas (playing a Composite Character based partly on Cat People producer Val Lewton) and Barry Sullivan spend a scene or two working on a B-movie called Cat Men.
Similarly, Jaws also used this trope as a loophole to film a movie about a shark attack virtually without a shark, due to the ceaseless problems with their mechanical substitute. Given how bad the props are in the sequels, the wisdom of this move is all the more apparent.
Robot Chicken, in a parody of the reediting of the original Star Wars trilogy, had a sketch where Steven Spielberg announces his decision to redo the special effects in Jaws. The results are not pretty, to say the least.
Alien pulled the same trick; the director realized that while H. R. Giger's design was awesome and the creature did look scary in glimpses in the dark, it ran the risk of looking fake if it was too visible. When the special effects caught up with the design, we got Aliens. Although not really, because Nothing Is Scarier fully applies here.
The design of the suits in Aliens were actually simplified (some only being leotards with bits and pieces of skin), not just to cut costs (because they needed a lot more suits), but to allow the actors a greater range of motion. In a well lit room the original would look far better, but because James Cameron kept them either in the shadows or moving too fast to clearly see, he gets away with it beautifully.
Bubba Ho-Tep of the eponymous Bubba Ho Tep was shown in shadows for the majority of the film; it was handwaved that he's so powerful that he sucks the energy out of light bulbs, so whenever he's walking down a hallway the lights in front of him will suddenly flicker out, etc.
Throughout most of the original Star Wars trilogy, Darth Vader's mask symbolized not only his evil, but the notion that his face must be so horrifying concealing it could not make it worse. The fannish disappointment was rife when the mask was finally removed, and revealed what one fan called "Uncle Fester with blue sparkles". This was probably an intentional subversion. The notion that Vader underneath the frightening armor was intentionally made to be a broken and pathetic individual has been noted in numerous interviews. In Lucas's own words, Vader is less a monster and more "a sad man who made a deal with the Devil...and lost".
In Children of the Corn (1984), He Who Walks Behind The Rows is never openly shown on screen. Presumably, the kids' murderous fanaticism was sufficiently horrifying that seeing the god/demon/spirit/whatever which they followed wasn't deemed necessary.
Subverted in Scrooged, where a character actually calls the bluff of the menacing hooded figure that claims to be a supernatural creature, and looks under its robes. The ghost is genuine, and the view is not pretty.
This was the original intent of Jacques Tourneur (who directed the above Cat People) in his 1957 Night of the Demon, preferring to show only smoking footprints and fiery clouds, but Executive Meddling had a rubber-suit monster put into the ending and the beginning. Still, most critics of this move agree it ultimately doesn't hurt the movie.
Used humorously in Beetlejuice when he demonstrates to Adam and Barbara that he can be scary. Something happens with his face, but we only see him from the back. In a movie filled with fun creepy special effects, the best one is the one we have to imagine.
This trope can apply to mortal humans, too. In Road to Perdition, Al Capone is deliberately kept off-camera to evoke a sense of mystery and dread about the most powerful criminal in Chicago, and the power that rests behind Frank Nitti.
The monster from Cloverfield is never directly seen until the end (and even then, it isn't that clear). We see glimpses of it at times earlier in the movie. There is, however, an official toy release of the monster showing its full body.
Similarily, the alien from Super 8 (made by the same creator) is never directly seen until the end (it is very clearly shown). You can look closely to see its reflection sneaking up on someone the second night. Somewhat subverted in that it isn't really evil, and just wants to go back to its home planet.
Death from Final Destination, whose presence is usually indicated by things like shadows, gusts of wind, ominous phrases, etc. Its true form does show up in one of the spin-off novels.
In The Evil Dead 1981, and for most of the second (up until the end), and for the entirety of the third again, we don't actually see the spirit that is chasing the characters through the woods; it's represented almost entirely in point-of-view shots. What makes this so weird is that the characters being chased will often look into the camera, scream, and flee, but then the camera will cut to another angle (not a POV shot) and the characters will seem not to be running from anything at all.
The Cabin in the Woods both subverts this trope and plays it straight: we get to see almost all of the monsters in their full glory, under bright lights and for extended amounts of time- but the most we ever see of The Ancient Ones is a giant fist raising out of the ground a the very end of the film.
One monster we don't see is simply known as "Kevin". That is the only thing we know about it.
This is the main point of the first Cube movie. Apart from Worth's connection to the titular cube, no information whatsoever is ever given about the titular cube's creation, purpose or nature. The sequels give us progressively more information.
An example of Ultimate Evil appears in the first book of the series, Flight From the Dark. If Lone Wolf ends up in the Graveyard of the Ancients, he'll stumble upon the tomb of an ancient king. If you hand him an Idiot Ball and he opens the sarcophagus...
You are in the presence of an ancient and timeless evil, far older and stronger than the Darklords themselves.
Revealed in the remake to be Naar, the King of the Darkness, the true Big Bad of the series, and literally the Ultimate Evil of the setting.
The stories ofH.P. Lovecraft used Ultimate Evil quite a bit; sadly, movies and TV shows based on said stories don't use it nearly enough.
Lovecraft himself is speculated to have been parodying overuse of the concept in the story "The Unnamable", although it's hard to tell since he always wrote like that. It's definitely parodied here.
Ghatanothoa in "Out of the Aeons" was a kind of meta-example. It wasn't just that the readers weren't ever "shown" it (the narrator gave a partial description but didn't think he could even try to really explain what he had caught a glimpse of), but the real catch was that within the story, you really, really wouldn't want to see it. Just the sight of Ghatanothoa would turn a living human being into a petrified but living mummy. If you were magically warded against this effect, you might still die.
The Crimson King, Big Bad of Stephen King's meta-continuity among his novels, possessing various incarnations across dimensions, such as The Man Behind TheBig Bad of The Stand, is constantly said to be the horrific source of all evil. However, behind-the-scenes Villain Decay sets in, and by the time he's revealed, he's a gibbering old man in a red cloak, who attacks the hero with weaponized Harry Potter toys while continually screeching "Eeeee!" and is then erased by Patrick. Given the absolute terror he inspires in his subordinates (some of it due to firsthand experience), there has been elaborate Fanon created to explain this inconsistency.
The degeneration of the main villain fits in with the overall theme of The Dark Tower, where everything is breaking down. Things fall apart, the center cannot hold...
Just look at the Gunslinger Born Prequel comics—the Crimson King is this scary spider-demon-thing that is eating a person.
Arguably, Pennywise the Clown from It. The forms that ARE seen are based on childhood fears. The giant spider form at the end, although weak-sauce to the extreme, was meant to be the most terrifying of the forms that humanity can safely comprehend. Beyond that, madness ensues: a secondary human villain Pennywise/It manipulates into bringing It a hostage looks at Pennywise when It's not wearing a form ("It did not bother to dress when at home.") and promptly keels over dead.
The Lord of the Rings, the eponymous villain Sauron is this, although he's not the biggest bad in The Verse. He's mentioned often but never actually appears, deliberately, to heighten the sense of his unfathomable, mind-breakingly evil power. He is, however, given some description in supplemental material, and going by those it's better that we don't see what's really behind all this craziness.
In the film adaptation, Sauron was given a full costume for the prologue, and was even intended to appear in the climax and duel Aragorn, before filmmakers realized how goofy that would be and digitally replaced him with a big troll. Still follows the trope though, in that we never see what he looks like underneath his armor.
The Silmarillion suggests that it was difficult for him to take physical form, at least without The Ring's power.
In the film he was never seen during the Third Age, although the giant fiery eyeball was mistakenly identified as his physical form by some viewers, including the 'Sauron blogger' who stated "I am not an evil lighthouse."
The Minotaur in House of Leaves. In reality, the Minotaur isn't so much a character as it is a concept invented by characters journeying through the house to explain the uneasy feeling that they're being watched, followed, and hunted down by some horrific creature. Tom Navidson even calls it "Mr. Monster" at one point. It is only called the Minotaur by Zampanˇ, who later struck through every passage containing that title. The strike-throughs are actually provided by Truant, who reconstructed the passages after Zampanˇ attempted to destroy them. On at several occasions, he succeeds, most notably on pages 372-373, the former of which contains the phrase [2 pages missing] and the latter of which is a series of XXXXXXXX interrupted only by one word and one partial word, though the footnotes survived.
In Beyond the Deepwoods, the first book of The Edge Chronicles, the Gloamglozer is handled this way... but according to its descriptions, seems to be a fairly underwhelming bogeyman not much worse than some of the threats you actually do see. In an inversion of how this usually works, when it actually shows up toward the end of the book, it turns out to be something far, far worse: a grotesque and malevolent trickster with more than a little in common with Satan.
"In Machen, the subtlest story The White Peopleis undoubtedly the greatest, even though it hasn't the tangible, visible terrors of The Great God Pan or The White Powder."(to Robert E. Howard, 4 October 1930)
Alan Dean Foster's Humanx Commonwealthuniverse features a classic Eldritch Abomination as its Ultimate Evil — a galaxy-sized region of space in which no matter or radiation exists. Moreover, it is sentient and mobile, traveling across the universe in search of new galaxies to devour. It has been discovered by several species at various points in galactic history, even the most advanced of which could barely do more than find a way to flee. Naturally, Flinx, the protagonist of the series, is the Chosen One who is said to be the key to its destruction. However, as scary as the concept is, the thing never actually gets to our galaxy before Flinx manages to destroy it, leaving its implacably hostile nature something of an in-universe Take Our Word for It.
Played with by both main villains in Mistborn. The Lord Ruler is kept off page for most of the first novel, building up an air of mystery and fear about him; as a result, even though other main characters have met him before, Vin is stunned the first time she sees him and realizes he's a pretty ordinary-looking man. Later on, the realBig Bad, Ruin is portrayed for the first part of the third book as a completely inhuman force of nature. Later, it starts interacting with mortals in suprisingly humanlike fashion, using images of people they've known as its avatars. Vin speculates that this is just a mask, though and she's proven right when she becomes a god herself and sees Ruin in his true form. What little description the reader gets could easily be summed up as "Eldritch Abomination", proving that while the heroine can now face the villain on his own terms, he's still brain-breakingly horrible to mortals.
In The Wheel of Time, the Dark One fits this trope perfectly. It's a nigh omnipotent evil god that has existed since the beginning of time and is the ultimate cause of all the conflict in the series. So far it's still mostly sealed away from reality, and even if it does break free, it's been implied that it probably doesn't have an actual physical form. The only time anyone has encountered it directly is when it communicated mentally with one of the Forsaken. Even then, we only hear its voice, and that alone was enough to make the person hearing it weep from a combination of agony and ecstasy. Even when partially freed it takes the form of a vast void rather than anything humanlike.
The First Evil in Buffy the Vampire Slayer was a type of Ultimate Evil, as it was shown briefly onscreen three times, but for the rest of the time, we never got to see it directly.
And was very disappointing when it was finally seen, thus proving the point of this trope.
The First Evil actually shows up often in the last season, but usually is taking the form of other characters, which comes off as legitimately creepy. When it just goes for a random demon face it loses some of its edge.
The Wolf, Ram, and the Hart, aka the Senior Partners of Wolfram&Hart from Angel are a great example of Ultimate Evil. A powerful and ancient cabal of demons that are the true power behind the series' main antagonists, they are never seen or even heard once. The demon that appears for the Review was just possessed by one of them. Yet the series makes their influence an undeniable and terrible thing. By the end of the series, they ultimately prove to be an unstoppable force of Evil that Angel and company can only fight, but never defeat.
The Source on Charmed. A good example of what's problematic with showing the Ultimate Evil, as well — after several seasons of only being mentioned in passing he's finally revealed as a mysterious cloaked figure. With each sucessive appearance, the Source gets more stupid looking and more like a traditional Big Bad, until finally he's killed off and replaced with new Big Bads.
Similarly, the aliens in The X-Files were, for the entirety of the first season, represented by slo-mo and flashlights.
The Family Channel had a short-lived series called Scariest Places on Earth which would use a night vision camera to capture the horrified expressions of those visiting the eponymous places and seeing the eponymous scary stuff, but that was it. Short-lived because nobody who watched the show once was stupid enough to want to watch it twice.
The new series of Doctor Who season 4 episode "Midnight" has a chillingly effective Ultimate Evil. Unlike all of the Doctor's other adversaries, it has no shape or form and is only known by its influence on others. The Doctor proves to be utterly mystified and helpless against it, and were it not for a Heroic Sacrifice by the tour guide, it would have succeeded in killing the Doctor. In its one appearance, it evokes the same fear from the Doctor that the Doctor usually inspires in other alien menaces, such as the Daleks.
Babylon 5 gives us the Shadows, who are seen, but relatively rarely. They look like giant, really nasty spider/mantises, but they're usually invisible—saving on the CGI budget and adding to the fear factor: a Shadow could be anywhere, lurking, spying.
The Vorlons, who eventually prove to be evil (or rather, Lawful Blue to the point where it's indistinguishable from Evil), are only ever seen in their encounter suits—again, their natural form is too CGI-heavy to be used that much.
The monsters under Calvin's bed are rarely seen (apparently because they shrivel up and die upon exposure to light). Sometimes, though, you'll see parts of their body in the dark, which don't look terribly cuddly.
Old Harry's Game: More like He Who Must Not Be Heard but the most frequently mentioned of the damned outside of the main characters is Adolf Hitler, yet he never has any speaking roles, is never spoken to, and with the exception of one scene where he is explicitly in disguise, is never even spoken about in such a way that one can infer his presence.
Pale Night, a demon lord from Dungeons & Dragons fits this trope. She appears as a ghostly woman wearing a shroud. Her true form is so horrifying, though, that reality itself rejects it; the shroud is not hers, apparently, but something the multiverse forces on her. (This is implied to be because Obyrith demons themselves are chaotic beings of entropy and madness; the reason for their hideous forms is because the, for lack of a better term, intelligence of the Abyss is forced to adhere to the rules of a lawful universe to bring its servitors into being. Pale Night's true form, though, managed to break those rules.
Her deadliest attack is the ability to suppress her shroud for an instant. Unlike almost every other example in the game, if you succeed on the Will save against this ability, your character is considered to have NOT comprehended what he saw, and blocked it out. Whereas if you fail they understand what they see and die instantly. If the character is ressurected, they will have no memory of what was seen.
Eberron has Kyber, the Dragon Below, who is one of the three beings from the beginning of time and now is the underworld. Same goes for Eberron, who is the world, and Syberis, who is the Sky, but they are not considred to be evil.
DND 3.5 supplement Lords of Madness gives vague description of beings that predates even the Aboleth, all of which are Lovecraftian entities that aren't even given statistics (unlike the aforementioned Pale Night), only how their influence shaped Aboleth societies. Aboleths are creatures for whom the writers had to invent a new word to describe: unhuman.
Not on the table, where they've basically been torn out of space and rammed into an airtight liquid metal skin. In their natural form they operate on a scale so large they were surprised when they found out that planets exist, let alone the little noisy things on them.
Yawgmoth, Big Bad of Magic: The Gathering's Weatherlight saga and the biggest villain the game has ever had, has never been depicted on any card. Even the tie-in novels are vague about his actual appareance.
An example in Fate/stay night; the heroes eventually find out that the MacGuffin they are fighting for was actually corrupted some time in the past and has become the home of Angra Mainyu, the Zoroastrian Devil. He is a being that is 60 billion curses personified and the antithesis of human goodness. And he hates back. The only thing we get to see is basically pure evil that is leaking from it, and it is implied that it has no 'real' shape. Except in the Heaven's Feel route, where it finally manages to manifest itself as a vaguely humanoid tangle of limbs and eyes. Luckily, it does not succeed in being properly born before it is obliterated.
And then in Hollow Ataraxia the trope is subverted. Angra Mainyu didn't exist until the Grail created him in accordance with the "wish" of the people who martyred Avenger: For there to exist an ultimate evil which they could blame for their own sins.
King Stan in Okage Shadow King is trapped in the form of a shadow for 95% of the game, citing that the entire world will shake in terror once he regains his True Form. It turns out to be less than impressive (although that chin is pretty scary).
Demonica of Stretch Panic is a monster so horrifying that merely seeing her causes Linda to die of fright. You must prevent her from entering the shack you are inside by following her shadow in the windows and attacking through the entrances she tries to use.
Giygas, the Big Bad of EarthBound, is an Ultimate Evil in similar ways to Cthulhu. And he's a rare case where he's finally revealed, and he's not only still terrifying, but probablyeven more so than he was before, due to how bright and kid-friendly the game was beforehand. This was probably more creepy for those who played the original MOTHER 1 where he was just a kind of creepy thin alien in a fancy fishbowl. All of his battle messages read "You Cannot Grasp the True Form of Giygas's attack!", and he is shown on-screen as a swirly red... thing. It's implied that this is not his true form, but what the Devil's Machine turned him into. Ironically, if you're wearing the Franklin Badge, you can grasp the true form of one attack, as the Badge's effect will reflect the attack back, revealing it to be electrical in nature.
MOTHER 3 has a time-traveling Porky. Just like Giegue/Giygas, one of his battle messages reads as "...?! What did Porky do...?!".
The player may discover that the Spathis' next-door neighbors are avatars of a real Ultimate Evil from another dimension. Hilarious as it would be, there's no way to point this out to the Spathi in the game.
The Watchers in Drakengard are made out to be ineffable and all-powerful by their servant, solidifying their position as the Ultimate Evil in the game. Except when they appear near the game's finale, they take a form that is indeed horrifying and morbid. Part of it probably comes from expecting the writers to play this trope straight, and the other part comes from the symbolism latent in their appearance.
Subverted in Darkened Skye, where the Big Bad, known as "He whose face must not be glimpsed" and universally feared by all, is ultimately revealed to literally be a tiny maggot. As the heroine puts it "He Whose Face Must Not Be Glimpsed? That's because he's too small to see!".
Deliberately or not, this might be a Shout-Out to Captain Marvel's enemy, Mister Mind. When he first appeared in the 1940's, it was over a year of comics before he appeared as anything but a voice over a radio, sending his Monster Society of Evil to wreak havoc. When he was finally revealed, his true form was... a superintelligent alien caterpillar about 4 inches long, wearing glasses.
The Dark Master Malefor of The Legend of Spyro series was not seen in the first two games (Except in animated cut-scenes which are not very representive of his real appearance) or heard, until he finally appears at the very end of Dawn of the Dragon, fufilling the trope completely. And he actually is every bit as horrific, powerful, and monsterous as he'd been made out to be. He's a purple dragon like Spyro, but he's far larger than normal and looks like a dragon straight out of the pits of Hell. He's also an Omnicidal Maniac whose sole goal is to destroy the world, and he comes so close to succeeding the world is already starting to break apart when Spyro lets loose a World-Healing Wave.
The eponymous Siren. You hear its cry — something like a distorted, unearthly air raid siren, in a play on the dual meaning of the word — but you never actually get to see it. The Sorting Algorithm of Evil skips right over it, taking you straight from the shibito to Datatsushi, The God That Fell, the creator of the siren, the shibito, and the red water.
Word of God is that the siren is just the sound of Datatsushi, but this contradicts the game itself; a secret cutscene shows the fall of Datatushi and the first appearance of the siren, and there, the cry of the siren and the cry of Datatsushi are clearly two entirely different sounds, the siren responding to Datatsushi's scream.
In Knights of the Old Republic II, you never actually see Darth Nihilus's face. The only scene where his mask is removed is done by a different character and his corpse is destroyed before you can look yourself. According to other sources, Nihilus is actually dead, and just takes the form of a mask and cloak through the force.
Kreia implies that he has, through eons of hate, malice, dark side power and soul draining entire species, become literally nothing but Evil with a lightsaber- making him possibly the only villain to ever hate himself out of the laws of reality.
Inverted in Riddle of the Sphinx: when you finally look inside the Ark of the Covenant, all you see of the Ultimate Good is blinding white light.
And in the fourth Zork game, they introduce the Ur-Grue, which is the progenitor of all grues and is capable of creating an aura of utter darkness around itself. Ya know how Grues don't show up if there's light? Yeah. He doesn't have that problem.
At the end of Commander Keen 4, Keen is shown what his new enemies the Shikadi look like. All we're shown is his face going through horrified expressions. This game is shareware and its full version is free; naturally the player is promised that they'll get to see what the Shikadi look like if they buy the next game. Apparently they don't look all that special.
The menace in Dark Fall: The Journal is never seen, although a monstrous figure does appear in ancient engravings and Room Full of Crazy art. Lost Souls, a direct sequal, forgoes even this much, using an enigmatic symbol to represent the entity's presence and power.
Silent Hill. The town itself. Especially in Silent Hill. This particular game is where this trope gets applied the hardest, as the town is unquestionably malevolent, and capable of shaping itself to inflict the most pain possible on its victims. However, despite multiple possibilities being offered, nothing ever really confirms for sure what makes the town the way it is, or why it does it. There's even some speculation that the God of the cult that lives there is actually also an illusion the town has created to inflict more suffering on the world. This probably done on purpose since ultimately, the town would be less scary if we knew why it was the way it is.
This is invoked in Arcanum: Of Steamworks & Magick Obscura in regards to Arronax. Arronax was a very real and very dangerous elven mage way back in the Age of Legends, and was sealed in the Void for his crimes. In modern times, he's treated as the ultimate evil, turning him into a folklore symbol instead of something real. This allows the dark elves to infiltrate the Panari religion, which was tasked with maintaining the seal that keeps him from returning, and trick them into ignoring their duty.
Depending on how you view her, the titular Villain ProtagonistSaya in Saya no Uta could count. All the viewer gets to see is her almost angelic-looking human form that only Fuminori can see (at least, angelic-looking compared to all the revolting Meat Moss that constantly assaults his vision,) and the occasional tentacle from her true form, which has driven everyone who's seen it insane (unless they were already insane.)
In Kaspall, a box shaped robe with one arm and a cane becomes horrific this way. Of course, knowing the things that it DID to its victims helps.
Schlock Mercenary features a species of Dark Matter entities that fit this trope. They're unable to directly interact with baryonic (i.e. 'normal') matter in any way, and therefore cannot be seen, heard, or in any way detected or interacted with... Except through manipulation of gravity (a mode of communication that mostly involves horribly tearing apart whatever they're interacting with like it was tissue paper). They appear to be none too fond of us baryonic entities, insofar they take pains to 'communicate' a lot when we end up disturbing them.
Homestuck's requisite example is Lord English, the Eldritch Abomination summoned by the death of the universe so he can feed upon its remains. He isn't constrained by things like time, though. In fact, he's already here. However, in the intermission between the 5th and 6th Acts, he does appear.
In the few pictures you see him, he's always slightly out of focus and difficult to see amongst the trees. We can't even see his current form properly.
Lo and behold the concentrated abomination that is Popsicle Pete!
SCP-055 is the most mysterious and potentially dangerous SCP contained by the Foundation. Its only known property is that it somehow erases any other information pertaining to it from all records and memory. As a result, no one remembers when or how the Foundation first acquired it. While it's entirely possible that the SCP is otherwise completely harmless, the Foundation isn't taking any chances and treats it like any other Keter object.
It is possible to remember what it isn't. Which somehow makes it worse.
And what, so far, do people remember it isn't? Round, safe, or contained.
Simliar to 055, SCP-579 is apparently so utterly horrible that the description is censored. All that's revealed is the containment procedures...which involve sealing the thing in an alternate universe (which is itself created by another SCP—something that's strictly forbidden by the Foundation's MO). And the protocol in case of a breach? Destroy the alternate universe. And if even that doesn't work? Well...
In the event of an unsuccessful Action 10-Israfil-B, no further action will be necessary. [DATA EXPUNGED]
Dungeons & Dragons: The Nameless One, boss of Venger, is a hugely powerful and evil being that destroys worlds only because it feels like it. Its body is permanently wrapped up in a massive tornado that reaches the clouds, and its real aspect is never shown. You can only see its glowing eyes.
Gormiti: The Lords of Nature Return gives us Obscurio, the supremely powerful leader of the Darkness Gormiti. While the toyline does feature a figure of him, he has not been seen in the series proper, only appearing as a spiritual entity which hides in a specially-forged crown that possesses Toby in Episode 6.
Similar to Darth Vader, Slade's mask is generally used as a symbol of absolute evil on Teen Titans. Unlike Vader, the viewer never gets to see what's beneath it- whenever it's torn off, what's revealed is either one of Slade's robot body-doubles, a quickly-concealed silhouette, or an undead skull. Of course, in the original comics Slade's a fairly ordinary looking middle-aged man, so the animated version probably shares that appearance.
Subverted with Trigon from the same series, who for his first few appearances is just a deep voice, glowing red eyes, and a silhouette, but is ultimately revealed in all his glory when he breaks through into the mortal world. Think Satanon steroids.
Nergal Jr. of The Grim Adventures of Billy & Mandy takes the form of a small boy with green eyes, as his real form shown off screen in his first appearance freaked out anyone who saw. However, in a much later episode when Billy angers him, he takes the form of a demonic creature for a few seconds. Whether this is his true form is uncertain, but it looks horrifying enough to be so.
Even that green-eyed boy form is one he stole from the first person he met when he came up from the core of the planet. His original form is not shown except that horrifies the child (who is never heard of again in the show).
XANA from Code Lyoko takes this trope Up to Eleven, to the point it doesn't even have a face or even body to begin with, being an AI. For the whole series, the only thing we saw of it was its mooks, its attacks and a mysterious Faceless Eye like symbol.
Mentioned by name in Transformers Prime by Agent Fowler to describe Unicron, who in this version is the Earth's core. That is, the Earth accreted around the dormant body of Unicron.
in Ed, Edd n Eddy Eddy's brother is kept from being seen, his presence only indicated by the stories Eddy tells, and his legacy only shown by the frightened Eddy, and the fearing Rolf. When we do finally see him, we see that he does mostly just look like Eddy, but coupled with his behaviors and the wildly frightening evil smiles and rather evil expressions he gives everyone, it's clear he is by far the greatest villain above the rest the show has to offer.