Diabolus Ex Nihilo

""Arthur! Monkey out of nowhere!""

- The Tick

Sometimes you have a goal in mind for your story, but you have no idea how to get there. For those times where the end justifies the means there is the Diabolus Ex Nihilo or "Devil from Nothing". The Diabolus Ex Nihilo is an enemy so foul, so horrible, and so evil that it needs no Backstory or reasoning. It just appears from nowhere, does its job of shaking things up and antagonizing the heroes, and then promptly dies. The Diabolus Ex Nihilo may get a back story in the future, but it would just be an exercise in retconning.

This trope can often appear in origin stories where it's more important that the characters are introduced than that they do something sensible.

See also the Giant Space Flea From Nowhere, which serves roughly the same function in Video Game gameplay -- a boss that pops up at the end of a level for no narrative reason and with no explanation, just because there's supposed to be a boss at the end of the level or the game.

Compare the Generic Doomsday Villain, who has no clear motive for their actions other than being "evil," even if they do in fact have backstory or context. Contrast Outside Context Villain, where the mystery behind a villain's origin, motive and abilities are the source of their threat. Consider Boring Invincible Villain, where them just showing up exemplifies It Got Worse. A Diabolus Ex Nihilo used by a writer to get out of a corner may be an example of Chandler's Law.

Yes, this may also be a Diabolus Ex Machina if it succeeds in doing some damage. See also the Anthropic Principle where the appearance of an otherwise unexplained baddie forms the premise for the entire story.

See also Ass Pull.

Anime & Manga

 * Digimon Adventure: Apocalymon, the final villain, literally shows up out of nowhere in an outer space-like battlefield, and is fought and defeated within the course of two episodes. Apparently this was actually a case of Executive Meddling, and there was supposed to be far more leadup to Apocalymon, as well as his relation to the Dark Masters.
 * In 02, Daemon and his followers appeared randomly, had incredibly vague motivations (they want the Dark Spores... for some reason) and are defeated in a few episodes after causing some pointless mayhem. They are then quickly forgotten.
 * In Frontier, after seemingly defeating Cherubimon and destroying his fortress, the heroes fight an IceDevimon who was apparently sealed inside since he was too powerful for even the Three Holy Digimon to defeat. The lack of hinting towards this and his relatively quick defeat suggests that his only purpose was to fill up half an episode.
 * Walpurgisnacht from Puella Magi Madoka Magica. It breaks the rules the other witches follow, with no explanation given for why or how. This is even lampshaded: Supplementary material describes her as the witch of stage setting; she exists so the story can happen.
 * Pretty Cure All-Stars movies seems to have a tendency to use this trope gratuitously, due to lack of connection between individual Pretty Cure series. The latest of which, in DX 3, is the entity simply known as Black Hole. It's best described as The Heartless of every Pretty Cure villain, ever.
 * Kain, the villain the the Tenchi Muyo! movie "Tenchi In Love".

Comic Books

 * Superman: The Man of Steel #17 introduces the most (in)famous Devil from Nothing: Doomsday. He basically emerges from the bowels of the earth, squishes a bird, and then goes on a killing spree for no good reason. He's pretty much there just to kill Superman and kick off the "Death" and "Rebirth" story arcs.
 * Darwyn Cooke's Justice League: The New Frontier spends so much time dropping obscure DC character names that it never creates a fleshed out back story or motivation for "The Centre"--a giant living island of dinosaurs determined to cleanse the Earth and then the Solar System. Ironically, the Centre itself is from an obscure source--Dinosaur Island, a Lost World featured in several "Weird War Tales" comics. Its existence was never previously explained, and Cooke decided to reinvent it as an alien menace.
 * It seems to briefly explain its motives by possessing an Expy for Dr. Seuss, who writes it down in a story before killing himself; it reveals that it's a prehistoric entity which tired of humanity's noise and yearns to leave Earth to explore the solar system. This will require exterminating humanity for ... some reason.
 * Its plan was to wipe out humanity so the Earth can heal itself from all the damages humanity has done.
 * The Big Bad in DC's Final Crisis, wasn't Darkseid. It was a Multiversal Vampire. This means exactly what you think it does: he eats stories. If you didn't read the Final Crisis: Superman Beyond tie-in, he seemed to come out of nowhere. Writer Grant Morrison did, however, intend for Superman Beyond to be an integral part of the story, and it is included in the trade editions of Final Crisis.
 * A Godzilla comic had Big G be sent back to the time of the dinosaurs, where he was attacked by a giant dragon... thing that was there for some reason; he manages to just barely beat it before being returned to the present.
 * The feral vampire hag that turned Cassidy in Preacher (Comic Book). She appears biting his neck, gets shot by his brother, and falls back into the swamp, never to be seen again.
 * The vampire that turns Lord Andrew Bennett, title character in I... Vampire. He turns up while Bennett is out riding, mesmerises him, gives a little rant about hating Bennett's positive outlook on life and wanting to show him the dark side and generally screw his life up, bites him, and then promptly gets staked by him. Then Andrew makes it home, filled with angst over his new condition, his wife agrees to be turned by him so that they can spend eternity together, she promptly turns evil, decides to take over the world, declares Andrew to be her enemy when he tries to talk her out of it, and flies off to begin her campaign. And there's your central plot and background set, now on with the episodic story.
 * Brightest Day had . It's mentioned a few issues earlier that a Dark Avatar will rise, but the White Ring is vague as to exactly what our heroes will be facing, so it certainly felt like one of these.
 * In Green Lantern: Rebirth, Hal Jordan's 90s Face Heel Turn is explained and excused as a result of his possession by Parallax, a "Fear Entity" and literal Giant Space Flea From Nowhere. Parallax has since become the Big Bad of a crossover event, one of the cornerstones of modern GL Continuity, and was depicted as the giant floating CGI head of Clancy Brown in The Movie.
 * The Joker is this; his backstory was introduced much, much later than the character and even in there, he outright stated that's probably false. Despite that he's still managed to rack up a body count any villain worth their salt would be jealous of, and is the person regular criminals tell stories of to scare one another.

Film

 * The Whale Probe in Star Trek IV the Voyage Home is never given any backstory or origin. It's the quintessential Big Dumb Object that exists to make Jim Kirk's court martial more of a hero's welcome. (The probe was eventually given a back story in the book Probe.)
 * Justified in Spock's exact line, "Only human arrogance would assume the message 'must be meant for Man'." One might imagine it was quite chatty with the cetacean life it was sent to contact.
 * Star Trek Nemesis: title is pretty much the description of the writer's intentions: make a bad guy that's bad enough for Picard. All of Shinzon's anger should be directed at his creators/torturers on Romulus, but then Picard wouldn't give a darn. So Shinzon needs to attack Earth with no motivation except to be Picard's Nemesis.
 * His anger is directed towards the Romulans, but it's pretty unsurprising that he harbours some bad feelings towards the man in whose image he was created. Attacking Earth is more to with his Reman upbringing since it's implied they are basically a race of blood knights, and it serves to show up the Romulans who have made several attempts to defeat Earth over the centuries and always failed. He wants to make a statement as the new leader of the Romulan empire.
 * The Fifth Element introduces The Great Evil, an angry, black sphere that doesn't even have a proper name let alone a motivation. But how would Bruce Willis find a cute thousand-year-old alien girl to nail without it?
 * The shark from Jaws has no reason to be so big, eat people, and sink boats. But thank goodness it is and does because Roy Scheider and Richard Dreyfuss are a match made in heaven. Of course, how much motivation does a shark need? Occasionally animals realize humans are pretty easy to kill and start eating them, they tend to keep doing so until they die.
 * Krull: The Beast appears from outer space, invades the world and randomly crashes wedding parties all in the name of being bad. While the backstory of the Cyclops indicates that the Beast has done this sort of thing on other worlds before, there's otherwise no backstory for the Beast itself explaining where it came from and why it decided to go about invading worlds.
 * In The Dark Knight The Joker has no Start of Darkness, no backstory (none that you'd believe, at any rate). He's just there at the start of the movie to wreak havoc, havoc and more havoc, just 'cause, which actually serves to make him even scarier than he might be otherwise. As Alfred puts it: "Some men just want to watch the world burn."
 * The ending of the American Astro Boy movie. Just as the day is saved and everyone is celebrating, a giant sun-shaped alien with tentacles that shoots lasers out of its one giant eye attacks the town. No reason, no foreshadowing, someone just shouts "Alien!" and it's there.
 * It was probably one last Shout-Out to the Artificial Sun from one of the earlier Astro Boy series.
 * Many Werewolf Movies, such as An American Werewolf in London, Ginger Snaps, and Wolf, in which the protagonists are bitten and become werewolves, do not go into very much detail, if any at all, in explaining where the original werewolf came from. This is because it is rarely very important to the story, the focus is on the protagonist's reaction to their condition.
 * Frankenstein Conquers The World: The "International" cut climaxes in a fight between Frankenstein's Monster (in city-stomping Kaiju form) and a giant octopus who suddenly shows up out of nowhere. Not only that, but it's a famous Japanese Mountain Lake Octopus, as most of the action takes place in the "Japanese Alps." Although it was filmed at the request of the American distrubutor, he apparently felt the end result was just too silly. The scene was cut from both the Japanese and American versions, but for reasons unknown, was kept in the "International" cut (English language, but for territories outside America.)
 * Monster X/Keizer Ghidorah from Godzilla: Final Wars. He just appears out of nowhere (Though it's implied he was inside the meteor "Gorath" that was heading to Earth), fights Godzilla to a draw, transforms into his more-powerful form, nearly curb-stomps Godzilla to death, and then is ultimately defeated after Godzilla gets a power boost.
 * The murderous robber dressed like Santa Claus from the beginning of Silent Night, Deadly Night.
 * The killer in HellBent is given no origin or motivation, never has his name or actual appearance revealed and isn't even mentioned in the credits. Also, making a literal example of the trope, he's dressed as the devil.
 * The Thief and the Cobbler: The Mighty One-Eye, and the entire race of monstrous One-eyed men he is the leader of (simply called the "One-Eyes") simply appear into the film without any kind of backstory. They want to conquer and destry a Golden City, also without any explained motivation.

Literature
""Some have said that she ended long ago, when in her uttermost famine she devoured herself at last.""
 * The Silmarillion by JRR Tolkien: Ungoliant; she is said to have "descended from the Outer Darkness, maybe, that lies in Ea beyond the walls of the World." She destroys the Two Trees and almost eats Morgoth. After breeding with lesser spiders, she just... wandered off.


 * The History of Middle Earth gives more details, with Ungoliant originally written as an Eldritch Abomination 'personification of primeval night' before the world was made, and later this being changed to her being a Maia and former disciple of Morgoth who grew strong enough to equal him in his long years of imprisonment. Also, Ungoliant was originally planned to re-enter the story -- Earendil slew her in one of his adventures on the way to Valinor. It's likely this would have ended up in The Silmarillion if Tolkien had ever finished that part of the story.
 * With the exceptions of Blaine the Mono and Andy the Messenger Robot (Many Other Functions!), pretty much every robot in The Dark Tower. They show up, kill, maim, or psionically alter someone, then are either destroyed or returned to hibernation.
 * "The One" from Animorphs. It is a literary Giant Space Flea From Nowhere, introduced in the very last book after the Yeerks are defeated. There is no explanation of where it came from or what it wants, it merely becomes the remaining Yeerks' new god, assimilates Ax, shapeshifts to a bunch of random things to freak the heroes out and gets its spaceship rammed. K. A. Applegate's comments seem to indicate that she just wanted the good guys to get screwed by a new war and created a Diabolus Ex Machina to let them go out in a blaze of glory.
 * Whatever it was that sent Jake to a Bad Future, as some kind of test, in The Familiar, the definitive BLAM Episode of the series.
 * In the Warrior Cats series, the Big Bad Tigerstar had nine lives at the beginning of the (then) final book, The Darkest Hour. In order to avoid making him seem like a pathetic weakling, the authors had a random cat called Scourge show up, kill Tigerstar all nine times, kill the protagonist, Firestar, and take over the Clans. And then Scourge died.

Live Action TV

 * Star Trek: The Next Generation's pilot episode introduces "Q", a god like entity that begins harassing the crew seemingly for the sake of it. He simply appears on the bridge and creates unnecessary drama. While he eventually becomes a beloved character by the end of the series, I dare you to see how he was important to the plot of the pilot and realize how tight the adventure would have been without him. Word of God says that the pilot was originally going to be one episode, but Executive Meddling forced it to become a two part episode. So Q was made up to fill the extra hour.
 * Star Trek the Original Series had the Doomsday Machine from the episode of the same name, a giant weapon that flies through space eating everything it comes in contact with, up to and including entire planets. Spock believes it came from another galaxy, and Kirk theorizes it may have been built as a form of Mutually Assured Destruction in a war, intended as a bluff or deterrent but ultimately used, however the true origins and purpose of the machine are never revealed.
 * The enemy from the Doctor Who episode "Midnight" -- we never see its true form, or learn its name. It just appears out of nowhere, possesses Sky, places the Doctor and everyone with him in grave danger,.
 * Considering how often the Doctor exhibits an encyclopedic knowledge of...everything...throughout his adventures, many count the episode as one of the eeriest in all Who, to the point of That One Case.
 * In Kamen Rider Decade: All Riders vs. Great Shocker, there's King Dark, the Big Bad of Kamen Rider X, who only rises after all the other villains are killed. It was never hinted anywhere in the movie that he would even appear at all. Since there is already a personification of the Great Leader, recurring Bigger Bad of the Showa era in, it means that there are two Great Leaders running amuck. He was probably just included to give an excuse to use the giant Kamen Rider J (the movie did say All Riders).
 * And then
 * The creators of Merlin promised in an interview that the end of season four would end with an intriguing Cliff Hanger in which a previously seen "evil" character would reappear. Most fans bet their money on Mordred. However, it turned out to be something completely random. In an early season four episode Merlin discovers a dragon's egg, hatches it, and calls the baby dragon Aithusa, said to be a symbol of the forthcoming Golden Age of Camelot. The audience was given absolutely no reason to believe that Aithusa was anything but a harmless baby dragon, who is not seen or mentioned until the end of season four, where it appears in the forest to heal an injured Morgana for no apparent reason and fly off again.

Mythology

 * In Hesiod's Theogony--an early Trope Codifier of Greek Mythology--Typhon is one of these. He shows up out of nowhere to wreck havoc after the Olympians win their war against the Titans and Giants. Zeus Curb Stomps him and the world is finally at peace. Some later retellings either edit this bit out for being too random or write the battle as a more grandiose affair.
 * Various tellings of the story change a bit of that; Typhon is discribed as a child of Gaia and Tartarus, set against Zeus by Gaia when she realized his rule wouldn't be much better than his father's. The battle was less a curb stomp, with Typhon having the edge early on(ripping out Zues' tendons) before being defeated by having a mountain dropped on him.

Tabletop Games

 * Legend of the Five Rings' notorious Hidden Emperor story arc featured Goju Adorai. He appeared halfway through the arc which continued to focus on other evil characters such as the Kolat, Onnotangu, and the possessed Toturi. Adorai finally shows up during the climatic battle at Oblivion's Gate as the leader of the Lying Darkness's forces where he is defeated with virtually no information ever given about his back story or motivations.
 * Arguably, the Lying Darkness as a whole.
 * People have spent years thinking of ever-sillier ways to defeat The Tarrasque, but what the hell is it?
 * By definition, any RPG modeled after the Cosmic Horror Story will tend to feature these.

Video Games

 * Jenova in Final Fantasy VII appears from the sky two-thousand of years ago and murders most of the planet's population. After being dismembered and frozen she is revived in modern times and starts all over again. No motive or origin is ever given.
 * Sephiroth hints that the cycle of parasitic destruction is Jenova's true purpose: it crashes into a planet, sucks out the Lifestream energy, and then leaves to crash into another planet. However, it's very vague at best. Sephiroth's goals may be his own, rather than an extension of Jenova's. However, in Advent Children Sephiroth does explicitly claim that he shall sail the Cosmos to find another planet to find a shining future, like his mother once did. So maybe he really is following Jenova's Goal after all. As usual of the trope, the explanation comes 10 years after the game and from a different writing team.
 * In Dirge of Cerberus, Omega Weapon's purpose is similar to Jenova's: to take the life energy of the planet and find a new planet when planet-shattering catastrophes happen.
 * Necron in Final Fantasy IX. The last boss of the game, it is basically the god of death, and where he comes from, how you get to the area he resides in, or why he wants to kill you all is not even close to explained.
 * Sulphur from Phantom Brave is this monstrous demon thingy that wants to kill everyone. Nobody knows why, nobody asks why.
 * Chrono Trigger: Jenova's grandpappy, Lavos: it  falls from the sky in the distant past and (after an initial firestorm) proceeds to wreck the place.
 * And then there's Chrono Cross, where the final boss  Oh, and then he
 * The Dark Star in Mario & Luigi: Bowser's Inside Story--someone accidentally dug it up one day, realized that whatever the hell it was, it was bad news, and immediately called upon the royal family to seal it away. Fawful thinks that it might be useful.
 * The titular mothership of the Fallout 3 add-on Mothership Zeta deserves mention. Without any previous foreshadowing, the Lone Wanderer is abducted by the aliens and must spend the next several hours (days, in game time) fighting his way through the mothership just to get home. At which point, the entire experience is never mentioned again. What exactly the aliens were doing with all the people they've been abducting over the past thousand years or so is left vague at best.
 * One must bear in mind that the aliens in Fallout are basically the Western equivalent of the Silent Hill UFO endings: completely out-of-place weirdness done as a joke that became a running gag in the series. To drive the nail home, the UFO stuff in New Vegas only appears when you have the "Weird Wasteland" trait active; this also allows such things as the Holy Hand Grenade.
 * In that respect, it's basically a Big Lipped Alligator Moment in relation to the game. Nonetheless, in game lore, it is possible the Alien Technology is the reason for Cyrogenics, Lasers, Plasma Weapons, and others.

Web Comics

 * Parodied in Adventurers--possibly even a direct parody of Necron--when, after finally defeating the distinctly non-Space-Flea Final Boss and fleeing his collapsing lair, the party is suddenly faced with Necrevil, "the embodiment of evil itself!"... and they just keep running.
 * Eternion on his first appearance is also an abrupt Giant Space Flea From Nowhere, but he becomes a recurring villain and arguably has more of an impact on the plot than the Big Bad himself.
 * While he's decidedly more human than most examples of this trope, this guy still qualifies by dint of location.
 * Morthol Dryax in Problem Sleuth, who pops up, is soundly defeated 22 pages later, and isn't mentioned anywhere else. Fluthlu counts as one too, and appears shortly after beating Morthol Dryax. It's also never mentioned again in the series, apart from a Mythology Gag or two in Homestuck.
 * Homestuck kind of plays with this trope. The nonlinear nature of the story means that for the reader it doesn't come out of nowhere, but from the perspective of most of the trolls

Western Animation

 * Teen Titans
 * Slade, whose backstory and motives were never revealed, despite the fact that he was the Big Bad for the first two seasons, the Dragon with an Agenda in the fourth, and had cameos throughout. The heroes, especially Robin, speculate as to who he is, but nothing concrete is ever reached.
 * Also, the Monsters of the Week from the episodes "Stranded" and "Things Change" similarly come out of nowhere and, though the characters wonder out loud what they are, are never given even a vague explanation.
 * Also Red X has no explanation or origin for his appearance, and no back story. He just appears out of nowhere and makes trouble for everone. Beast Boy tries to speculate, but Raven says it could be anyone and no one else bothers speculating.
 * Dr. Claw in Inspector Gadget is a faceless enemy that is constantly plotting against the titular hero. His organization seems to have no other purpose than to "Get Gadget". Claw's face was not even revealed until nearly twenty years after the show's run ended. Except on a toy.
 * Transformers: The Movie features Big Bad Unicron who drifts out of the vastness of space and starts eating things and making bargains with Decepticons. Though he appeared from the vacuum, he certainly didn't vanish afterward though.
 * Further incarnations were slightly more careful with this. Armada hinted he was in the background for a while, Energon followed as a sequel, and Transformers Prime dropped hints about him every now and then before he appeared for real.
 * Thundercats: In the series finale, after Lion-O fights an epic final battle against his Arch Enemy Mumm-Ra, the Ancient Spirits of Evil randomly throw "their champion," a giant warrior named Pyron, at Lion-O just to fill up the last couple of minutes or so of the episode.
 * Although not many of the villains in The Powerpuff Girls have elaborate origins, the nanobots in "Nano of the North" seem especially noticeable in this regard. They're a swarm of microscopic robots that come out of a cloud that forms over Townsville but nothing else surrounding it, start devouring all the carbon to make more of themselves, and demonstrate some kind of group intelligence in the way the cloud changed to focus the forces onto the Powerpuff Girls. Although they're all destroyed by the end of the episode, no clue is given as to their origin or why they were targeting Townsville specifically, and indeed no one even asks.
 * The evil alien force from "Forced Kin" qualify as well.
 * The Loc-Nar from Heavy Metal is never given a backstory, but it's the embodiment of Pure Evil that drives all the vignettes in the movie.