Ultimate Evil

""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.'"

- Stephen King, Danse Macabre

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 POV 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). 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.

Anime & Manga

 * Mao-chan parodied the trope with an alien that was too cute to be shown, as it caused 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.
 * Berserk
 * Baccano. Subverted. 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.
 * Ultimately subverted 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 finally makes his debut...

Comicbooks

 * 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, exagerated 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 labloratory explosion but by the demon Mephisto scratching his face (the result of said experment, 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.
 * 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".
 * They may very well be full-fledged Eldritch Abominations.
 * 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.
 * 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.

Gamebooks
"You are in the presence of an ancient and timeless evil, far older and stronger than the Darklords themselves."
 * An example of Ultimate Evil appears in the first book of the Lone Wolf 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...


 * Revealed in the remake to be.

Film

 * A magnificent example of Ultimate Evil is The Last Wave, which is all about the end of the world and about doom. There isn't a single effects shot.
 * Ultimate Evil is the entire premise of The Blair Witch Project.
 * This is because the makers couldn't afford a really scary monster effect or suit. It ended up working better than if they had. There were, unfortunately, a few toy releases of the witch which . To be fair, these toy releases are not official.
 * 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.
 * "You missed me, you dried-up douchebag!"
 * Alien pulled the same trick; the director realized that while HR 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, 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 Ridley Scott 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 Hotep 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".
 * Galactus in Fantastic Four: Rise of the Silver Surfer is only shown as a massive cloud of smoke. This is likely because his common depiction in the comics is, frankly, rather silly looking. This helps because Galactus doesn't actually have a set form in the comics; different species perceive him differently, because his true form is incomprehensible to lesser beings. He is, for all intents and purposes, a god (being older than the universe). The "silly-looking" humanoid in the purple & blue armor is done for our benefit. Beta Ray Bill, for example, sees Galactus as a giant starfish-type creature.

For the movie, the massive storm cloud is his most basic form. The director (Tim Story) realized that having the comic book Galactus appear on-screen for only two minutes would be anti-climactic, to say the least. However, being a fan of the comic books, he included references that only people who have actually read the comics would understand: when The Cloud passes by Saturn, it casts a shadow in the shape of his helmet; when The Surfer confronts him in the eye of the storm, the flames form an outline of his head.
 * SKYNET, the Big Bad from the Terminator franchise, has never been depicted on-screen (except for in various non-canonical video games and the Universal Studios Terminator ride). Justified in T3 when it turns out that SKYNET is, in fact, the Internet. An avatar of SKYNET appears as a character in Terminator Salvation, played by.
 * In Children of the Corn, 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.
 * In Disney's Bambi, the Ultimate Evil known as "Man" is never shown on screen.
 * Word of God states that the human that shot Bambi's mother was, in fact, Gaston.
 * 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 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.
 * 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
 * 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.

Literature
""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)"
 * The stories of HP 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 The Big 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, . 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 eveything 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 Lord of the Rings, where the eponymous villain Sauron is literally the Ultimate Evil, 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.
 * 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.
 * It's debatable. He lost his ability to assume a 'fair form' after dying in the sinking of Numenor (Atlantis). He lost his physical shape again at the end of the Second Age, though IIRC, the Ring was cut from his hand after he was dead. He refomed a physical body during the Third Age, first becoming known as the Necromancer of Dol Guldur, then openly declaring himself when he returned to Mordor. He lost his ability to assume a physical shape at all only after the Ring was destroyed. See Gollum's statement to Frodo: "There are only four fingers on the Black Hand, but they are enough."
 * That line could be interpreted either literally or metaphorically.
 * The ring was cut from his hand at the culmination of the siege of Barad-Dur, while he was most certainly still corporeal. The movie portrayal of the severance with the broken Narsil is something like how it was written (although it doesn't go with Gollum's statement about just one missing finger). Note that Morgoth also gained permanent scars earlier on.
 * 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."
 * T in House of Leaves. In reality, t isn't so much a character as it is a concept invented by characters journeying through the h 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 t by Zampanó, who later struck through every passage containing that title, and this would be reflected in this Wiki if strikethroughs hadn't been disallowed because of abuse.
 * The strike-throughs are actually provided by Truant, who reconstructed the passages after Zampanó 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.
 * A series of short stories by Robert W. Chambers leave us (and a young fan named HP Lovecraft) wondering, "Just what the hell is the The King in Yellow?
 * Chronicles of Prydain Big Bad Arawn was never seen (or even described) in his true form.
 * Wrong. Well, mostly wrong..
 * Just after the stoic Doc Savage escapes through the entrance of a strange underground cavern he looks back.
 * 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.
 * Ultimate Evil is the subject of Arthur Machen's short story The White People, with elements of The Fair Folk. As written by Lovecraft:


 * Alan Dean Foster's Humanx Commonwealth universe 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 real Big 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.
 * The Graveyard Book: We are the Sleer...
 * We do manage to get a glimpse of it/them,however.
 * 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.

Live-Action TV

 * 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 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 antagonist Wolfram&Hart, they are never seen or even heard once. . 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.
 * And according to Illyria back when she was ruling the world they were just small fries barely worth acknowledging. Makes you wonder...
 * The Senior Partners started out weak, but used Wolfram&Hart to increase their power and influence by feeding on humanity's evils.
 * 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, 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.
 * Reavers in Firefly are never seen on-screen; only their ships and the after-effects of an attack are. This got turned on its head when they got revealed in Serenity and proved, once again, why Tropes Are Not Bad.
 * Smallville portrays Darkseid as this.
 * 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.

Tabletop Games

 * Pale Night, a demon lord from Dungeons and 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.
 * Gwydion, a powerful Sealed Evil in A Can from the Ravenloft setting, is never seen or described in the published products,.
 * 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.
 * Warhammer 40000. The four Chaos Gods and the Emperor of Mankind all get this treatment to varying degrees. The Cosmic Horror-flavored C'tan, sadly, do not.
 * 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.

Videogames

 * An example in Fate/stay night; the heroes eventually find out that 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.
 * And then in Hollow Artaxia.
 * 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, for example, all of his battle messages read "You cannot grasp the true form of Giygas's attack!" While he is shown 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.
 * In MOTHER, the first game of the MOTHER series does have a physical form: the form of his attacks is still "inexplicable", but the above "implications" are merely the result of uninformed speculation. It doesn't change the fact that this must have been quite a shock for Japanese players who were expecting to see the original Gyiyg only to be treated to the swirling nightmare Giygas becomes.
 * Mother 3 has a time-traveling . Just like Giegue and Giygas, one of his battle messages reads as "...?! What did do...?!".
 * Parodied in Star Control II: The cowardly Spathi live in perpetual fear of the Trope Namer, the ULTIMATE EVIL!!! (emphasis theirs, every time). They know absolutely nothing about it, and have never even seen it, because (they claim) it always lurks just outside the range of their most advanced sensors. This is, of course, further proof of its nefarious intent.
 * The player may discover that the Spathis'  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.
 * During the course of Jade Empire, what has been done to the Water Dragon is described as pretty much the Ultimate Sin.
 * Oh, and let's not forget what said Ultimate Sin entailed back at the Spirit Temple. With the Water Dragon and the monks gone, -something- slipped in through the cracks of reality. The exact nature of this entity is never revealed; its simplest form is a giant, glowing green orb. It mostly works through demons, and your final confrontation with it is just copies of you. Because it doesn't belong in the natural order of the world, the gods are powerless against it, and it won't give you so much as a name. Oh, and it's the living embodiment of hate, fear, darkness, and suffering apparently. It should tell you something about Jade Empire that this thing is a side-quest.
 * 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 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 pretty much 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.
 * 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, all you see of the   is blinding white light.
 * "It is pitch black. You are likely to be eaten by a grue."
 * 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.
 * The Vasari from Sins of a Solar Empire are running away from a terrible, nameless evil that destroyed all of the inner colonies of their once-great empire. We never learn much more about it, because in their eagerness to get the game out, the developers forgot to include a campaign mode, and as a result the plot ends at the beginning of the game and (until the expansion) the lore serves only as an explanation for why the sides' units look and act the way they do.
 * 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.
 * 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 2. In 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, nothing ever really confirms 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.

Webcomics

 * 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.
 * The Monster in the Darkness of Order of the Stick qualifies as this; so far, he has only been shown in complete darkness.
 * Subverted in that
 * The creator has also stated that he will be revealed eventually, and that he's actually a pre existing monster. Whatever he is, he is confirmed to be ugly.
 * 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.

Web Originals

 * Slender Man seems to tip its toes in this, depending on the source. While Slendy is universally portrayed as an abnormally tall man in a business suit with no face, there have been hints that he is some sort of shape-shifter, and that this is not his true form.
 * 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.
 * Zalgo.
 * The "evil force" in Greek Ninja. At first, no one knew who or what was behind it.

Western Animation

 * Dr Claw, the villain from Inspector Gadget was never shown on the original animated series. For the first movie, he was played by Rupert Everett, but was clearly meant to be a completely different villain. An action figure of Dr Claw was made, when it was revealed that he
 * 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.
 * He Man and The Masters of The Universe and She-Ra gave us Horde Prime, the man behind both Skeletor and Hordak. All we ever saw of him was a greenish glow and a huge mechanical fist.
 * 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 orignal 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 Satan on steroids.
 * Well, actually, we see Trigon in the very first season. Or at least, we see Raven's Inner Darknesss and it looks exactly like him. So more of an inversion, or perhaps a zig-zag, with avatar of villain revealed-> true form of villain concealed-> true form revealed.
 * Nergal JR of The Grim Adventures of Billy and 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 that trope Up to Eleven, to the point it doesn't even have a face or even body to begin with, being an IA. For the whole serie, the only thing we saw of it was its mooks, its attacks and a mysterious Faceless Eye like symbol.