The Obsolete Space in Dennou Coil. Bits and pieces of outdated virtual environments (often representing buildings and locations that have been demolished in the real world) mashed together, filled with obscuring mist and Living Shadow-like Illegals.
Earlier in the franchise there was also the Dark Ocean in Digimon Adventure 02, gloomy home to digimon that... weren't really digimon, in thrall to a CthulhuExpy.
In Hayao Miyazaki's Spirited Away, the seemingly abandoned theme-park/town undergoes a similar transformation at sunset as the streets and buildings come alive with eerie spirits. It isn't actually evil, though; it's merely where kami and ghosts go to relax, and naturally is only active after dark.
Constantine has the titular protagonist travel to Hell that looks like the immediate aftermath of a nuclear strike of modern Los Angeles. With demons.
Carnival of Souls may be one of the first films to use this. The protagonist would at times slip out of the perception of those around her, losing all sound as well. Other times, near the titular Carnival, the games and rides would come alive, as would ghoulish dancers who beckoned her...
In The Dark (2005), Adele takes a trip into Annwyn which was certainly a Dark World even if it was meant to be a sort of celtic hell.
Occurs in a New Zealand short film, The French Doors. A man moving into a new home buys a pair of French doors and installs them. The next morning, he wakes up to a beautiful day with sun shining though the windows... except through the French Doors, which remain in complete darkness. Going out through the doors, he finds a darkened version of his garden and realizes that he's not alone.
When they are summoned to Earth, The Cenobites from the Hellraiser series create and/or inhabit these- for example, they can be filling empty rooms with meathooks and torture devices.
Jacob's Ladder seems to revolve around a character's frequent shifts from his ordinary life into a nightmarish dark world filled with demons. It eventually turns out that he's been Dead All Along, and both worlds represented his refusal to let go of his earthly cares and embrace the afterlife. This film unsurprisingly served as a primary inspiration for the Silent Hill series.
"The Gloom" or "Twilight" in Night Watch (2004) and Day Watch (2006). (Based on the novels by Sergey Lukyanenko). This world is a black and white version of our world, with some useful magical properties, but it drains the person entering of their life energy so it's used at high risk.
In Silent Hill (2006) Rose travels to the town of Silent Hill which alternates between "normal" and "dark world" throughout the film.
In the Super Mario Bros. movie, the parallel dimension city is essentially the Dark World to New York, though Mario can't much tell the difference. It's suggested in supplementary materials this was done consciously, Koopa looking at New York when he passed through the portal several decades earlier and getting ideas for how to rebuild the other world that he had just taken over.
The afterlife for people who killed themselves in Wristcutters: A Love Story is essentially a mild version of this trope. Its just like the regular world, only less colorful and more depressing, and nobody ever smiles. And there's a black hole under the front passanger seat of the protagonist's roommate's car.
In John Metcalfe's 1920 short story The Bad Land a new arrival at a British health clinic soon discovers an abandoned road that gradually leads him into an eerie, twilight version of reality that only one other resident has experienced. That resident has a theory that pockets of "the bad lands", as he calls them, are erupting and invisibly spreading from central points all over the world, but the story leaves open the possibility that both of them are mentally unstable.
This is essentially what became of the Domain after the Skulltaker (not that Skulltaker) offered up the skulls of the Kurgan, Hung and Tong chieftains at the Black Altar in Blood for the Blood God by C.L. Werner. This is because the original leader of those clans used those lands to bargain with Khorne for power, but instead of using it to glorify the God of War as he promised, he used it to establish his own little kingdom. In essence, Khorne was just taking his due. The moral of this story, as said by the pants-shittingly frightening Norscan Khornate is: don't try to cheat a god.
Coraline: The Other World is a perfect example of this. Interestingly enough, it first tries to look like an improvement over the real world.
Henry Kuttner wrote a novel with the very apt title The Dark World about this trope. The dark world of the story is an alternate version of Earth which has drifted into a fantasy world, although the magic is given a scientific explanation.
Alan Dean Foster's Into the Out Of, where the Out Of is the parallel dimension where the demons are coming from. The heroes have to go there to close the gates.
J. R. R. Tolkien's The Lord of the Rings has the world of the ringwraiths, which one can enter by putting on the One Ring. This trope applies especially to the movies, where the wraith world is depicted as a spooky shadowland.
Shadow in Tim Waggoner's Like Death.
Some Philip K. Dick stories feature dark versions of the "real world" of the protagonists, such as Martian Time-Slip in which a powerful Martian colonist named Arnie Kott uses an autistic and/or schizophrenic boy and a Martian ritual to send him back a few weeks into his own past so he can make a business deal, but finds he is in a hallucinatory version of the past tainted by the boy's fearful fixations on entropy and death.
Graham Masterton's Mirror.
The first level of Twilight in Night Watch, which Others can enter by stepping into their shadows, is like this. The deeper layers don't resemble Earth at all - the second level has, among other things, three moons and much weaker gravity.
Technically, it has the same gravity as the rest of the world, it's just that gravity accelerates you at mundane world speeds, and you're moving much more quickly than that.
Also, at the second level, you can barely even see man-made structures. They resemble a dark mist, which you can fall through if you're not careful. Anton kills a low-level Dark Other on a TV tower by entering the second level and pulling the Dark Other down. When the Dark Other emerges back into the human world, he's hanging under the walkway with his fingers embedded in the glass floor.
Interestingly, in Last Watch, Anton notes that, as he goes deeper into the Twilight, it looks, at first, less and less colorful, but, at some point, being to look more and more colorful until the sixth level looks almost like the human world if humans had never evolved. Of course, this is because the Twilight is circular, and the seventh level is actually the human world.
Yami-gaia in Sailor Nothing, from the Japanese word for dark and Greek for Earth.
When it was first brought up in The Elfstones of Shannara, the Forbidding was never really seen, but it was given a vague description of being a dark and empty void to which the demons had been imprisoned. When we finally get to see it in the High Druid series, it was revealed to be a Dark World of the Four Lands.
Terry Brooks explored this idea even earlier in his Landover series, wherein it was revealed in the first novel, Magic Kingdom for Sale—Sold! that Abaddon is a Dark World of Landover.
The Territories in The Talisman is something of a smaller magical reflection of our world. The Territories themselves are an alternate reality version of America and as such there are many ways in which the two worlds mirror each other. One of the first is the town of Oatley and one of the last is the nuclear wasteland that is the Territories' version of the great plains.
In both the book and the film of What Dreams May Come, suicide victim Ann spends the afterlife in a dark world version of her own house. The similarity is enough to confuse her into thinking it's the same house.
In the first book of the Feline WizardsSpin-Off series, the underground city of the Children of The Serpent is also described as a twisted reflection of Manhattan.
Some of Thomas Ligotti's less dreamlike horror fiction has elements of the dark world, particularly "The Frolic", in which an inmate of a mental asylum describes a sort of ruined and rubbish strewn Dark World.
The titular dream-dimension-thing from "Vastarien" is a particularly surreal example, seeing as it is, essentially, the protagonist's vision of paradise.
The alternate Bruges in "The Journal of J.P. Drapeau" is certainly one.
The Charmed Ones had once traveled into a dark version of their world created to keep balance with their version.
Kamen Rider Decade has the Negative World, where the heroes arrive after helping out in the worlds of their nine predecessors. It looks like Natsumi's homeworld, but is full of human-hunting monsters and evil Kamen Riders like Ryuga, Orga, Dark Kabuto and Dark Kiva.
Considering that the Light universe in Lexx was ruled by a totalitarian theocracy led by an inhuman monstrosity, it's saying something that the Dark universe was actually worse.
Overall it's not completely dark, but Neverwhere does have spots that distinctly qualify, such as The Bridge of Night and The Beast's Labyrinth, and it does exist parallel to Earth.
One was featured in an ad for a hearing aid of all things. Showing a man walking with his granddaughter, who gradually grows fuzzy, before fading completely. The colors desaturate as he enters a dark version of the park, with no other life besides faint indistinct whispers on the wind. Probably the most nightmarish commercial ever seen.
Evil Hat Productions' Don't Rest Your Head is an RPG set in a world clearly inspired by the titular Dark City, and Neil Gaiman's London Below from Neverwhere; and which is reachable by the protagonist only after succumbing to destabilizing, long-term insomnia-fueled madness-inducing sleep-deprivation.
Dungeons & Dragons 3rd edition has the Plane of Shadows which fits this trope. The 4th edition replaced it with two such worlds: The Feywild, a more fey-oriented and wild version of the material plane, and Shadowfell, the bleakly depressing afterlife.
Exalted has the Underworld, a realm of stasis, death, and shadow brought into existence by the destruction of several of the Primordials who created the world, populated by ghosts, and ruled by the emissaries of the undead hulks of those same Primordials. It isn't always geographically coterminous with Creation proper, but it makes up for it in sheer creepiness.
The shadowlands are an example, too. Essentially, the Underworld overlaps with a part of Creation in a spot or area where gruesome deaths occurred. You might not even notice that you walked into a shadowland right away.
In the JAGS Wonderland RPG, Chessboard Two is like this, being a broken and deserted reflection of "Chessboard Zero" (the real world, or at least the world we live in) populated by... things.
KULT has the endless city of Metropolis and the nightmarish realms connected to it, filled with twisted mockeries of life and sanity. And there is also Gaia, the darkside of nature where "eat or be eaten" is taken to an extreme. Both of them are The True Reality, as opposed to the illusion of the "real world" that mankind lives in.
The New World of Darkness has a few of these: the Shadow Realm (an animistic reflection of Earth occupied by spirits), the Underworld (seldom glimpsed, but not a bright and cheery place), and the Hedge (the midpoint between Earth and Faerie, equally wondrous and dangerous).
The Old World of Darkness had the Penumbra and Dark Umbra. The Shadowlands from Wraith The Oblivion may be the best example — the wraiths gaze out on the same world as us, but everything seems to be in a state of decay.
And, in a meta example, the old World is written as a dark reflection of the real world, where extremes are more stark, gothic architecture abounds, and things are generally worse than in the real world. The new World tends to play it as the real world but with supernatural elements, which is a much different approach.
The Nightlands invading the real world is the central premise of Palladium's Nightspawn/Nightbane RPG (name changed after first printing for legal reasons). The Nightlands are ruled by demonic sorcerers and illusionists who keep the mindless doppelgangers of real world people as slaves. There is also The Dreamscape where nightmares are literally real, and can find their way into the real world.
Lord Entropy's Chancel in Nobilis is actually known as "The Evil World". Since the Darkest Lord is a Card-Carrying Villain, it lives up to its name - complete with monsters that die immediately if they undergo a Heel-Face Turn - and it can occasionally overlap with Prosaic Reality. As a general rule, just... don't go down Samael St. Augustine unless your will is up to date.
The Wretched Hives of Shadowrun's Sprawls can either become that way due to malicious city spirits, or the spirits can become disgrunted by urban decay. Anyone capable of astral perception may witness a Darker World than the usual City Noir.
Astral travel/perception in general has a lot of potential for this, in places where powerful spirits or elementals are hostile, territorial, or bound to guard from astral intrusion, even corporate facilities and datastores. The projected forms of Runners and corporate agents themselves take on surreal appearances. It's a different kind of warzone for the magically active characters.
In the Yu-Gi-Oh! card game, there is the Dark World archtype. All the monsters are Humanoid Abominations and the images on the Spell and Trap cards depict a hellish landscape with organic looking walls.
In Akrasia, if you go for some time without collecting pills in the maze, the maze goes from bright and cheery to dark and gloomy. And then a monster descends upon you, reversing your direction keys. You can easily return to the light world by taking a pill. Subverted in that the Dark World is actually the world that contains the exit from the maze; if you keep on taking the pills to stay in the non-dark world, you'll be unable to escape the maze (and will eventually die). The seemingly nightmarish world is meant to represent drug withdrawal in that it can be frightening at first and very tempting to flee back to the apparently much more pleasant world with just another pill, but that if the player resists the temptation to take more pills in favor of finding an actual way out, s/he successfully "kicks the habit" and is able to return home.
Near the end of Alan Wake, it's revealed that one of these sort of exists under Cauldron Lake.
Another early example comes from the NES A Nightmare on Elm Street game. While you're exploring the regular town, a sleep gauge slowly drains away. If you don't keep it recharged with coffee, you'll eventually shift into the dream world. On the bright side, you can then change into one of four superpowered forms. On the not so bright side, the setting becomes darker and more twisted, with water turned into blood and the enemies stronger and now bearing Freddy's features. And once you're in the dream world, another, unseen timer begins to count down until Freddy's Ironic Nursery Rhyme theme begins to play. If you haven't woken up by the time it finishes, you'll end up trapped in a Recurring Boss fight with Freddy himself.
The PC Game Blair Witch Volume 1: Rustin Parr uses this trope to help explain some of the movie's mysteries. The woods of Burkittsville contain hidden "paths", as described by Native American lore, that twist through different levels of reality. While the normal forest is safe enough, there are other versions of it, such as the crimson-lit "Red Woods" and the foggy, twilight "Blue Woods", each inhabited by its own monsters and bearing key geographical differences (for example, in the normal woods, Rustin Parr's house has burned to the ground, but it's still standing in the Red Woods). The worst of these worlds is the Black Woods, a snowy, pitch-black level of reality that "the Hecaitomix" has made its home.
Castlevania: Harmony of Dissonance features a Dark World referred to as Castle B, and you've been flipping back and forth between the two, though you won't find this out until halfway through the game. Castle B is exactly the same layout as Castle A, except with different enemies, bosses and items, but it's all wrecked compared to the pristine Castle A.
The alternate dark hotel from Trilby's Notes of Chzo Mythos.The same effect starts taking place in the Optimology basement in 6 Days a Sacrifice, though it serves no real gameplay purpose.
In the Constantine tie-in video game you keep alternating between the real world and Hell (which, as mentioned above, is a hellish version of the real world) to solve puzzles, leading to some ridiculous situations. At least one reviewer snidely remarked that this is the first game that literally makes you go To Hell and Back to open a door.
In the Dark Seed games, the Dark World is a twisted, desolate alternate version of Earth. It doesn't help that H.R. Giger designed it. It's subversion in that the evil aliens that are threatening the real world actually aren't from the Dark World and just recently conquered it from the mostly peaceful original inhabitants..
Deadly Premonition has the "red world", which you enter for combat sections and which overtakes the entire town after midnight. It's never really explained what exactly the world is, although it's pretty clear that it's not just York being crazy.
Amaterasu Server in Digimon World 3 is something like a Dark World version of the regular game world. Not really evil, just shrouded in eternal darkness. Comparitively, the Asuka Server where you start out in is always sunny 24/7. Travel between the two "servers" is done using a combination of 2 different secret (and dangerous) routes.
Disgaea 2 features an unlockable Dark World, all the regular stages are similar with much stronger enemies and a malevolent sun that screws with the players every few turns.
Dissidia 012 Final Fantasy, the setting of the game "World B" is a copy of "World A", the world of the original Final Fantasy, but is entirely desolate with different terrain and Mordor on the north-western continent.
Doom 3 has moments throughout the game where the hero seems to see reality change from the already wrecked, lifeless base into a blood-streaked, skeleton-littered nightmare world, only for everything to snap back to normal a second later. And the "Resurection of Evil" expansion has an NPC outright state that the Delta Labs area is phasing in and out of the Alternate DimensionHell, creating a more tangible Dark World where the two intersect.
Dragon Age: Origins, another BioWare product, has The Fade, which consists largely of flawed copies of material-world areas floating in a spooky void. This is Justified though, since the landscape of the Fade, with the exception of The Black City, is an ever changing reflection of the dreams of mortals brought into being by spirits and demons.
Dragon Quest VIII has a Dark Is Not Evil version, because although the world is a darker version of the normal world (and everything is black-and-white, with very little color being present in it, in contrast to the vibrant world the heroes came from) and the Sealed Evil in a Can the heroes are after hails from there, the people there are as friendly and helpful as in the normal world. Unfortunately, unlike most examples such as the Trope Namer, only a small part of this game's Dark World is visited (specifically, the Dark World's counterpart of an island the heroes visited; the heroes have no way of getting to the rest of the Dark World due to the fact that they have no way off the island since they only have a ship in their own world), even though the full world would have no doubt been very large like the normal world map.
The Red Night in 11eyes, a world categorized by its overlarge black moon and red sky. Electricity doesn't work in this world, all people except for the chosen six disappear, and horrific monsters roam the streets. Scary place.
Moonside from EarthBound, though it's just a hallucination caused by the Mani Mani statue.
Eden Eternal has two parallel worlds, one of which is the dark world.
In The Elder Scrolls' in-game mythology, the Oblivion realm of Molag Bal, Coldharbour, is this: it looks just like Nirn, but with every imaginable catastrophe and disaster rolled over it.
The freeware game Eversion starts out in a typically cheerful retro platformer world, but in order to progress you must "evert," traveling through a sequence of mostly-similar worlds that each contain crucial differences (i.e. whether particular obstacles are intangible, solid, or breakable) allowing you to navigate through a maze and collect all the gems. Once you get to the third world, it becomes clear that each World in the sequence is gradually Darker than the last - a "descent into Platform Hell" if you will.
Fable Heroes has a dark version of each of it's levels that you can play once you beat the light version of the game.
Fallen London, the titular city and the vast cavern of the Neath that surrounds it, It's dark, but also often whimsical and fun.
Final Fantasy II inverts this in the Soul of Rebirth bonus quest—the path taken in the quest is assumed to be the Jade Passage and Pandaemonium, locations in Hell. It turns out that the areas are actually Raqia and Arubboth, and the party is in Heaven, which appears as a mirrored reflection of Hell with a lighter color palette.
Dynamis. A dream world originally created by the avatar Diabolos to escape the Emptiness, other beings began to be pulled in, and turned it into a Beastmen-dominated version of Vana'diel. The area itself is warped enough that aside from temporary visits by players (even then, they need spiritual assistance to get in), if you get stuck in there, it's for good.
The Emptiness itself resulted in the creation of Promyvion — bizzare, half-complete mockeries of the vicinity of the crags that hold pieces of the mothercrystal. Promyvion is implied to be composed of fragmentary memories of decayed souls.
Finally, we have Abyssea, perhaps the straightest example of this trope: a depressing parallel universe where the Crystal War was lost horribly, the aforementioned crags are missing, and XP parties play somewhat like Dynamis runs.
Although it was only briefly glimpsed at the end of the first Ghostbusters movie (where it housed Gozer's otherworldly temple), Ghostbusters: The Video Game reveals that the ghost world is like this in spots where the dimensional boundaries have weakened, giving rise to a ghost world New York Public Library hovering in a cosmic abyss, and a ruined, cobweb-shrouded version of the Sedgewick Hotel.
Legacy of Kain: Soul Reaver had the "spectral realm" you had to enter to solve certain puzzles. The geography would often twist and warp, creating paths that weren't in the physical realm. Not to mention the soul-eating monsters that would hunt you down.
Blood Omen: Nupraptor's keep is in the shape of a skull, built on a cliff. Looking through one eye socket will show you the lush landscape below. Looking through the other will show you the world through Nupraptor's eyes, a dead, twisted land with blackened ground and lava for water. Kain remarks that Nosgoth doesn't need help to make its corruption apparent.
Another Legend Of Zelda example, of course, would be the encroachment of the Twilight into Hyrule in The Legend of Zelda: Twilight Princess. Any area under the sway of the Twilight looks essentially identical to its normal state, except that the colors are washed out and little bits of darkness constantly peel off of everything and float up toward the sky. Sapient beings are turned into spirits that look like little wisps of light, and non-sapient beings are turned into black, tentacly versions of themselves. And then there's the Twilight Realm itself, which we admittedly don't get to see too much of, but what little we do see seems to be a dark inversion of the regular Hyrule, complete with its own ruling Princess and peaceful inhabitants.
The Legend of Zelda: Skyward Sword has the Silent Realm, a blue-and-orange world that Link accesses by thrusting the Skyward Sword into specially marked spots. Because Link has to leave his sword behind to stay in the Silent Realm, he must rely on his agility to survive against the Guardians, monsters that can One-Hit Kill the player. The three Triforce pieces are located within as well, suggesting that it is or will become the Sacred Realm seen in later installments.
Mabinogi sets part of its mainstream story quest-chain in Tir Na Nog, a dark version of the Uladh region of the main world. It features prominently in the conclusion to the conclusion of the Generation 1 story. To most of the human NPCs, it is only a mythological paradise, which they may or may not believe in. Very few know the truth about it.
Vindictus, the prequel to Mabinogi, is about the battle to escape the Shadow World in order to reach the promised land of Erinn, where the original game takes place.
The Plane of Myrror in Master of Magic is a Dark world counterpart to the primary plane of Arcanus.
Super Meat Boy has darker counterparts to the "Light" worlds where the levels usually have a darker tone and are much more difficult.
Metroid Prime 2 takes this to the logical extreme with Dark Aether, which was created when a Phazon-infused meteor struck the surface of Aether and split the planet into two parallel dimensions. In addition to being populated by tougher monsters, Dark Aether constantly drains your health (ridiculously quickly at first, then more slowly once you get certain upgrades) if you're not standing in a safe zone generated by certain crystals.
One of the first video game appearances of a dark world is in the cult classic NES game Monster Party, a platformer that has the hero going to a planet of monsters to rescue a princess. The first level's set in a pastel Ghibli Hills world with rows of happy people in the background beneath a a bright blue sky. Halfway through the level, a bolt of lightning crashes, and everything changes: the people are rotting corpses, the world's turned to sickly shades of green and black, and the music's changed to a solemn dirge.
Although Outworld in the Mortal Kombat series is usually just Another Dimension, one arena in Mortal Kombat: Deception crosses into this territory. The Yin-Yang Island is a tropical island that's caught between dimensions: it shifts constantly from the sunny, sandy beach on Earth into a stormswept Outworld nightmare during the battle, with the palm trees changing into giant snakes and back, and the ocean either being clear and peaceful or murky and swarming with piranhas.
The Secret World combines this with Amusement Park of Doom in the Savage Coast area. Not only is Atlantic Park pretty horrible already (being overrun with zombies, like most of the island), there's also a shadow version of the park (entered by riding the ferris wheel) that players have to enter in order to hunt down the evil behind the haunted park.
Shin Megami Tensei III: Nocturne downplays it with the Mirage dimension in Kabukicho Prison. Visually, the lighting and mist actually make it less creepy than the real one even if it's upside-down. Then you see that empty rooms in the real world are torture chambers to drain Magatsuhi from Manikins in this side...
The original Persona has the two worlds coexisting beside each other. Early in the game, the city is replaced by a dream facsimile. The east side of town appears normal, while the west side is enveloped in darkness, indicating that it is under Guido's control. The previously-visited subways and shopping malls on this side have deteriorated into war zones; store mannequins wear bandoleers and riot gear, and everyone is now dressed in Mad Max punk fashions.
Persona 2 has the so-called "Other Side", which is merely the result of the heroes failing in their mission. When Nyarlathotep kills Maya Amano and destroys the world, Tatsuya implores his spirit guide, Philemon, to turn back time. The sequel, Persona 2: Eternal Punishment takes place in a parallel world in which Innocent Sin's events did not occur. Nevertheless, a few characters (JOKER in particular) remember the "Other Side", and imply that it still exists. The game's ending has Tatsuya, who also remembers the Other Side, swap places with his alternate-universe counterpart, allowing one Tatsuya to resume his life in blissful ignorance. The other Tatsuya returns to the Other Side to help rebuild.
Persona 3 has a "Dark Hour" between midnight and 12:01am when the Shadows come out to play: the sky turns a sickly green, the city's splattered in blood, a twisted supernatural tower dominates the landscape and most humans are turned into indestructible coffins. They're the lucky ones.
Persona 4 places all of its dungeons in a world accessed by sticking your head through a turned-off TV. Each new dungeon reflects how the victim of the month sees the world around him. The big winner of this is Magatsu Inaba, which is a dungeon based off of the actual town of Inaba.
There's a Dark Realm in the Devil Summoner:Raidou Kuzunoha games, although it's not quite as creepy and twisted as most of these. It's mostly just, well... dark, and full of demons.
In Shin Megami Tensei: Strange Journey, you eventually get the ability to "phase between dimensions" in certain areas, allowing you to progress and reach new areas. The Schwarzwelt, its name literally meaning "Black World", is also in and of itself a Dark World, with several dungeons being twisted parodies of various parts of the human world.
Silent Hill features multiple levels of this trope in play. Most games feature the main character flipping back and forth between the derelict, fog-shrouded town and its hideous, blood-and-rust Dark World form. The relatively more normal-looking fog-shrouded town itself may be a Dark World mirror of an ordinary populated town (Purgatory versus Hell), depending on the continuity and your preferred explanation.
The Meta-World mechanic from Ougon Musoukyoku has this kind of effect on the stages when used. Some of the Meta-World versions of stages are prettyfantastic.
the white chamber two main "dark world" sequences, complete with more terror than usual, Room Full of Crazy, and really, really weird reality warping. However, the "regular" station you go through most of the game in is also a dark world of sorts, you see what the station really is supposed to look like at the end.
The moving through "The Veil" in Wolfenstein (2009) can cause some objects to look very strange. Examples include all light sources emitting a blue flaming aura, and brand new fighter planes looking as if they had already been shot down before they've even engaged in combat.
Since the introduction of phasing in Wrath of the Lich King, World of Warcraft missions sending the player to such locations have become increasingly common. The Realm Of Shadows and the Emerald Dream, connected to death knights and druids respectively are both represented in-game as a shadowed and oftentimes corrupted version of the present world, roamed by nightmarish beings.
In Guild Wars: Nightfall regions can become "Nightfallen" when there is a strong enough connection between the Realm of Torment and Elona. These areas are heavily warped, have the same dark sky as the Realm of Torment, and are populated by demons and Margonites. Within the actual Realm of Torment several Nightfallen versions of zones from Elona can be found, which are even more warped.
Final Enter has a dark world called TAILSwith sealed monsters inside it ..it's the world that the protagonist wants to go to.
Tom Siddell loosely modeled this place after his hometown, Birmingham. Given the way he describes the city, he seems to consider Birmingham a Real Life example of this setting as well; this led to many of the fans referring to Zimmy's mind city as Sunny Birmingham or Zimmingham.
In Sinfest, when Slick wakes, finds his (evil, animated) reflection gone, and goes into the mirror, he discovers that it's this.
In Bogleech's Don't Get Spooked you will wind up in one of these if you get spooked 3 times. While the normal world is full of strange monsters, the ones here are so alien they can't even be communicated with and the walls and floor are made of meat.
The Slender Man's home dimension is sometimes described to be this.
In Marble Hornets Tim is briefly transported to someplace that probably qualifies.
The Halloween episode of Invader Zim has, appropriately enough, a nightmarish dark world born out of (or, at least, somehow connected to) the hero Dib's mind. Eventually the Eldritch Abominations lurking within it manage to capture and use Dib as a gateway into the real world... and the sight of the series' own Crapsack World sends them fleeing right back into the dark world.
The Real Ghostbusters did this prominently in one episode. Egon, Peter, and Ray are transported to a hellish alternate version of Manhattan populated by ghosts and malevolent spirits. Interestingly, physics seem reversed there - the visiting humans have supernatural abilities, whereas the native spirits don't. New York City proper has also been transformed into a Dark World version of itself in a few episodes.