A Sister Trope of Start of Darkness that involves a non-Badass normal taking numerous levels in badass, whether instantly or over the course of a story, thus becoming essentially nothing but a giant walking, talking bucket of Nightmare Fuel.
Through a series of unlikely coincidences, or through accidental or unknowing actions on the part of the heroes themselves, or as a result of things which should never have happened — and in most alternate timelinesdon't happen, but just barely by the skin of their teeth, do happen — someone who might have stayed an insignificant nothing is transformed into a nightmare that grows and grows, absorbing the power to rend civilizations to dust and bring universes to their knees.
For some reason, it seems that From Nobody To Nightmare villains are always more powerful, terrifying, deadly, and threatening than your typical Diabolical Mastermind. This may owe itself to the shock value of the idea that anyone and anything could accidentally become the Big Bad; it's much more comforting to believe that only long years of Card Carrying Villainy could possibly breed that kind of person.
Compare the Diabolus Ex Nihilo. If the character was already a bad guy previously but never a serious one, it's Not-So-Harmless Villain. Heroic versions may still find themselves wondering "Dude, Where's My Respect?". Frequently, the details of this process are uncovered in the villain's Start of Darkness. Also see Who's Laughing Now?.
Note that being a villain is not necessarily a requirement for this trope, even though most who fit the trope are. Anyone who is feared or dreaded by someone (even if the "someone" in question is the cruel and tyrannical government that the heroes oppose) can qualify.
This is pretty much the whole point of the Superfriends story The End, where the Legion of Doom become far, far more tormented, horrific and powerful beings than their previous laughably incompetent selves.
Nyx is an intriguing variant... She is forced to become Nightmare Moon in a physical sense, but doesn't really act evil at all, even though everypony expects her to.
In the grand scheme of things, this is also what happened with Discord. Twice, actually He was always a reality warping spirit, but of his entire race he was the weakest, and was slightly nicer than later in life. Then, in the Alicorn/Draconnequi war he both discovered his love of sadism and increased his power by eating his brother. Then he was reborn on Equestria, with no memory of his past existence, and became an odd but ultimately kindhearted being. Then his original personality resurfaced, and he became the Big Bad from canon.
Film Critique (aka the Pegasus Despot) from "Gaiden: 7 Dreams/Nightmares". Before the collapse of G2 society, he was just some fat nerd. Afterwards, he got his hooves on the Blue Shard of the Rainbow of Light (Laughter), which not only gave him the ability to control ponies, but also to restore power to his city, which allowed him to become its Evil Overlord. Then Patch shows up and steals the Shard back from him, leaving him to his Karmic Death at the hooves of his previously-brainwashed harem, begging like the pathetic little nobody he was.
My Little Castlevania has The Creepy Undertaker Dirt Nap. An outcast among ponies because of his special talent for burying the dead, he grows to hate the rest of Ponyville in turn. When he stumbles across Dracula's minions searching for one of his body parts, they sense the emptiness in Dirt Nap and offer him a purpose and a chance for revenge against those that scorned him. He accepts and becomes the host for the demon Aguni, before proceeding to burn down most of Ponyville.
In The Wizard In The Shadows Harry is technically already a nightmare - to the bad guys. He proceeds to become an even bigger nightmare. First, he's an eleven year old muggle raised kid. Then he becomes The Chosen One and a general badass. After his unwilling long term stay in Middle-Earth, he's a Knife Nut, a Master Swordsman and the terror of every bad guy short of Sauron himself - who is noted as being wary, though not in a personal sense, of what Harry is capable of - and has people speaking his name in tones of awe/fear from the tip of Northern Middle Earth to Far Harad.
In the fic itself, he proceeds to ramp it up even further, wielding Physical God level powers (though he was being powered up for those incidents) and is revealed to be a low level Reality Warper he disregards the basic laws of magic. And yes, only his finite power levels and lack of comprehension of exactly how powerful this makes him stops him being a total Game Breaker.
The same could be said for Lyrius, who was simply a teenage boy and a resident of a city in the path of her demonic army, who would have been slaughtered just like everyone else had he not had the courage to stand up to her, thinking he had nothing to lose. She was in no danger, but the display impressed her enough to offer him a place by her side which he accepted. He later became one of the most dreaded warriors of the Emperor of Shadow, and feared by the Queen's subjects almost as much as they feared her.
The WWE NXT season one rookies were just some wet-behind-the-ears, fresh-out-of-training nobodies — maybe something someday, but for now not worth much. Then they banded together to form The Nexus, and completely tore apart Monday Night Raw. For a few months after that, they were "the biggest threat WWE has ever faced" — bigger than The Alliance, bigger than the nWo, bigger than the McMahon-Helmsley Regime, you name it. Ironically, their undoing came when Daniel Bryan, the only rookie who actually had a good amount of experience before coming on NXT, and who had been expelled from the group for showing remorse, joined the WWE wrestlers in the fight against The Nexus.
The Miz. When he first appeared on television in the summer of 2006, it was merely as the obnoxious emcee for that year's Diva Search (which Layla El won, by the way). He eventually started wrestling, but almost everyone (except for Michael Cole, which Cole himself would Lampshade years later) thought of him as a joke who had only been offered a contract with WWE because he'd been on so many reality shows. (Outside of Kayfabe, of course, he'd been the second-place contestant on Tough Enough, but WWE commentators never bothered to point that out.) He remained essentially a "curtain-jerker" for his entire first year on TV, even getting eliminated from his first Royal Rumble Match after only seven seconds! It was only after he was drafted to ECW on SciFi in the summer of 2007 and paired with John Morrison in a heel tag team that he started to emerge as something resembling a threat. Fast-forward three years and several titles later, and suddenly Miz is "Mister Money in the Bank" and eventually "the most must-see WWE Champion in history!" And, of course, "Awesome."
Mark Henry spent the better part of 15 years as a midcarder, best known for his angle with Mae Young, and was considered an oft-injured bust who the WWE only hung onto to justify the untold millions they (over)paid him. When the company was short on monster heels, they turned to Mark Henry to see if he could fill that role finally. With strong booking that had him decimating upper midcarders and main eventers alike, coupled with a rejuvenated Henry in the ring and on the mic, he was able to finally be convincing as one of the most dangerous men in the company, if not THE most dangerous.
Cody Rhodes made his WWE debut back in July of 2007 as a generic babyface who teamed up with veteran Hardcore Holly to win the World Tag Team Championships later that year. But Rhodes would turn on Holly at Night of Champions in June of 2008 by revealing himself to be Ted DiBiase Jr.'s mystery partner. Both DiBiase and Rhodes would spend the rest of 2008 to early 2010 as Randy Orton's Legacy henchmen who did all of Orton's dirty work. By spring of 2010 when Legacy disbanded, many experts predicted that DiBiase would be the breakout star of Legacy while Rhodes would be stuck in WWE Superstars-land. But it would be Rhodes who would become one of the WWE's top heels as well as one of the longest reigning Intercontinental Champions while DiBiase faded to obscurity.
Triple H started off in WCW as just another long-haired meathead with a silly Wrestling Doesn't Pay gimmick during a time when these gimmicks were a dime a dozen. He got squashed by The Ultimate Warrior during his first WrestleMania match, then the curtain call happened... But by 1997, he was randomly paired up with fellow long-haired Kliq member Shawn Michaels to form DeGeneration X, WWE's version of the NWO. From there, he would go on to marry the boss's daughter both in storyline and in Real Life, win both world championships over a dozen times, and become one of Professional Wrestling's nastiest tyrants, earning himself monikers such as 'The King of Kings' and 'The Cerebral Assassin'.
Cyric of the Forgotten Realms was originally a rank-and-file thief in his guild, then an ordinary if somewhat amoral and greedy, mercenary. After the Time of Troubles he became the Prince of Lies, the Black Sun, the Mad God, the Lord of Three Crowns; an evil god even when compared to other Greater Evil deities, who controls the portfolios of Murder, Lies, and Strife (and for a while Death, Tyranny, and Intrigue as well). And the title "mad god"? That's an understatement. He makes Lloth look sane.
Lloth is merely so chaotic she schemes in several opposite directions at once, but her scheming is coherent. Cyric made rotting Moander at its final gibbering stages of degeneration look sane. Then again, probably making the Cyrinishad in such a way that in the process he had to be exposed to it himself wasn't the brightest idea to begin with.
The Archdevil Bel is probably one of the greatest examples of this. He started out as an ordinary lemure, a weak pile of goo that doesn't even have an intelligence score all the way up the promotion chain until he overthrew his boss and became the Archdevil of the first layer of hell. True it's the lowest ranking Archdevil there is, but tell him he's not a nightmare to his face and he'll probably kill you and use your soul for some evil deed.
And he's not even the only one. According to the second Fiendish Codex, ALL devils are like this, except for the few who are fallen angels or such.
Orcus deserves special mention too. Legend says he was originally an evil mortal, who ended up in the Abyss as he died, as a lowly manes. But he was cunning, and over several millennia, worked his way up, becoming stronger and evolving into more powerful demons. Presently, he's a demon lord, and is not only the number two - or three, depends on who's talking - ruler in the Abyss, he has more mortal worshippers than any other demon, possibly making him the one closer to gaining true godhood - his stated goal - than any other demon.
The Tau in the latter. When the mighty Imperium of Mankind first discovered them, they were no more than a simple plains-dwelling race that had barely mastered fire. Fast-forward six thousand years and they've assembled a multi-species empire capable of standing against the might of an Imperial crusade and defeating a Tyranid splinter fleet without a single lost vessel. However, they're still small-time on the galactic scale, controlling maybe a hundred worlds compared to the Imperiums one million. The main reason they've been able to survive is because they're located in the arse-end of the galaxy and generally not seen as a major threat to the Imperium (they managed to hold off a crusade for a time, but had the Imperium not been forced recall its forces to combat the Tyranids, they'd have been crushed eventually).
This could also apply to the Necrons in their own time (at least prior to 5th edition), who went from an insignificant, short-lived race orbiting a dying sun to an endless army of immortal killing machines possessing the most advanced starships in the setting.
Asrubael Vect, the supreme overlord of Comorragh, started out as nothing more than a slave. He managed to escape slavery and became a gang leader, and then slowly amassed power until his forces were powerful enough to be noticed by the noble houses that ruled the Dark City (though he did make sure to not become powerful enough to be seen as a threat by them). Eventually he manipulated events to cause the death of the leaders of the three biggest houses and leave their forces depleted and confused, then used the opportunity to seize power for himself.
Ghazghkull Mag Uruk Thraka; now, the most feared and powerful Ork Warboss in the galaxy, an Ork who brought a Hive World to its knees. Nine years before the "present", he was just a random thug who suffered a head injury and started hearing the voices of the Ork gods.
Every post-Heresy Space Marine and Chaos Marine. To become a Space Marine you need to be selected from the population of the Marine homeworld or a world it often recruits from. Every Power Armor- clad, genetically engineered killing machine of the Imperium was once a teenage boy on one of any backwater planets, a lot of them Death Worlds or Feral Worlds. These kids are badass, no doubt, but in the Imperium of a million worlds and trillions upon trillions of citizens, not so impressive.
In Exalted, this is often part of the backstory of Abyssals and Infernals. Infernals are offered their Exaltation by the Yozis after a deep, personal failure, and Abyssals only get the chance to become Abyssals when they're at death's door. The Prince of Shadows is a particularly good illustration of this trope as it applies to Abyssals.
A frequent factor in Betrayal at House on the Hill. That cute little boy who collects bugs? Just discovered a nest of Big Creepy-Crawlies and thinks they're so cool he's just gotta feed his new friends fresh meat. That sweet little girl with the teddy bear? Stumbled across a gateway to hell. The possibilities go on and on...
This trope can take place not only in the plot, but in the mechanics. Most games revolve around a haunt, or a time when some player turns on the others. The traitor and the others compete to fulfill different, opposed goals. Character stats in the game can go up and down. A character might start with only modest physical or mental stats, pass a few checks, Take a Level in Badass, find a magical item for a further buff...and then wind up betraying the rest of the players while easily the most powerful character in the game.
Delta Green features a few amongst the more human antagonists. One was once just a guy learning how to play guitar from some random drunkard. A pretty teenager from Ohio. A health food magnate. Dirt encrusted war orphans, one of them missing an arm. All of them viciously insane, completely amoral and enslaved to the will of powerful and dangerous alien entities.
One of the planets of the Screaming Vortex in Black Crusade is a dismal wasteland where nothing can be built, populated by permanetly depressed people. However, the inhabitants of the planet all harbour deep resentment and unspeakable desires that they're unable to act out on their bleak homeworld, and if taken off fromt he planet and allowed to finally act out their desires, they tend to become some of the most merciless and feared mortal followers of Chaos.
In the Ace Attorney series, Joe Darke was an ordinary office worker, until he killed someone with his car. This led to a chain of events that made him an infamous Serial Killer.
The beginning of Phoenix Wright: Ace Attorney has Miles Edgeworth. He's known as the Demon Prosecutor who is said to do whatever it takes to get his guilty verdict, including forging evidence (though Case 1-5 verifies that that particular rumor was untrue) and who never lost a case before going up against Phoenix. It's later revealed though that Edgeworth used to be a perfectly normal kid, albeit one very interested in being a famous defense attorney. Character Development in later games makes him less of a nightmare, though.
Fate/stay night's Sakura Matou goes from the cute girl with a jerk brother who gets kicked out of all the routes early, never does anything important, Cannot Spit It Out and more Shrinking Violet traits to an all powerful dark mage with the exact magic traits needed not only to be able to be the host of the (sort of) devil, but also express its power. And also to keep her mind (to a certain extent). Oh, and she can eat Servants or corrupt them as she pleases, have an immense Healing Factor and a bunch of other nifty moves. The one thing it doesn't do for her is improve her self image, causing her to believe that Rin (her sister) doesn't care about her (Rin's rather cold statements don't exactly help matters...), and to worry that Shirou might abandon her for Rin.
Avenger/ Angra Mainyu too. Started off as just an ordinary, run-of-the-mill Mesopotamian villager. After being randomly picked to be a scapegoat for 'all evils of the world' he ended up being summonable for the Holy Grail War, but even there he was still pathetic, being one of the weakest Servants imaginable. However his spirit then went inside the Grail where he made a wish to truly become the manifestation of evil for revenge on humanity, and that's the reason the Grail has been corrupted ever since.
In Nine Hours, Nine Persons, Nine Doors, Akane goes from being a sweet little girl to masterminding a second Nonary Game to get revenge on the four men that caused her death, as well as to get Junpei to show her the solution to saving her life.
Karnak from Dominic Deegan was a human orphan raised by orcs. Through a Heroic Sacrifice to save Miranda Deegan, he was transformed into a semi-demon and gradually slaughtered his way up the ranks to become the only Demon Lord in Hell.
Most Infernomancers in the series fit this trope as well. And Necromancers. And maybe Siegfried. Yeah, Mookie sort of loves this trope.
undyingUmbrage/Caliborn is, at first glance, a caricature of the Hatedom, a parody of bad fan-portrayals of Karkat, and yet another way to troll the fanbase. His first appearance is mostly composed of him pestering Dirk into drawing Paradox Space's tamest porn for him and making weird Saw references, and he mostly comes across as a misogynisticdouchebag who's too dumb to pose any real threat. And then he reveals that he just had his sister assassinated. It is now all but stated that Caliborn is actually a young Lord English.
...and now we have Aranea Serket, whose whiplash-inducing Face-Heel Turn is enough to break one's neck. Making it worse? The action that put her over the Moral Event Horizon was "healing" Jake's emotional trauma, allowing him to use his powers against everyone else in the session on her behalf—against his will. And the fallout? Considering he's :a Page of Hope, explicitly stated on several occasions to be one of the most powerful classes, not exactly pretty.
Also, Xykon goes from being an unfocused thug with no real long-term goal other than just wanton murder and destruction to an undead monstrosity that threatens the entire world.
Tarquin was once a small-time adventurer who tried to set up his own nation on the Western Continent. However, he joined together with his old friends and engineered a conspiracy which controls most of the Western Continent. He is now the de facto ruler of the Empire of Blood, as well as the other two Western Continent superpowers
The Snarl started out as a tiny piece of creation that embodied the pantheons' disagreements over how the universe should work. It was so insignificant that the gods either didn't notice or didn't care about its existence. Then it got bigger and meaner to the point that it could curbstomp entire pantheons. It is now supposedly the greatest threat to all existence, with the entire series revolving around efforts to control or seal it.
Pretty much the main story arch of Zebra Girl. the series began when a random magical accident transformed an ordinary tech support specialist into a demon with a soul. From those humble beginnings, she has gradually lost her humanity by inches, and has spent the past several months of the comic gleefully and unrepentantly terrorizing the inhabitants of her hometown as the living embodiment of fear. Basically, she has become a better-looking version of Freddy Krueger. With longer claws.
Coyote of Gunnerkrigg Court believes that etheric beings like himself only exist because of human belief. Assuming his theory is true, he was once an ordinary coyote until a delirious dying human saw him as a deity.
Doctor Horrible's Sing-Along Blog goes this route with the main character. In the prequel comic released, he was merely a geeky kid who was inspired by a Mad Scientist's moment of triumph to be a Supervillain (albeit one with good aspirations). He consistently remains unsuccessful at impressing his superiors and is eventually shoehorned into a situation where he must commit a murder or be executed for incompetence. Eventually with the accidental murder of Penny, the girl he loved, he becomes a true supervillain in both position and personality.
Survival of the Fittest provides plenty of opportunities for (mostly) normal high school students to become multiple murderers. J. R. Rizzolo, in particular, went from being a regular guy who liked playing Guitar Hero to a sadistic, deceptive loony who offed his former classmates without a trace of guilt.
Coyle Command found its origins as a laundromat. Its leader, Coyle Commander, had the job of opening and closing it every day.
League of Legends has Jericho Swain, the Master Tactician. His first record of being known in Noxus was when he walked into an infirmary with a severely mangled leg. No one but Swain (and maybe his bird) knows what happened, and he walked off afterwards with nothing but a cane in hand, refusing magical treatment, going off to join the Noxian military. Instead of being turned away for being a cripple, he tore his way through the ranks as a commanding officer, being promoted when demotions were asked for, and being decorated after nearly every battle. And even now, he still walks around with a cane.
Frequently used in The Gungan Council for villains. Better examples would be characters that managed to fly under the radar for a year or two before everyone realized how nightmarishly involved they were in all of the galaxy's problems.
A host of kids in the Whateley Universe who are nothing until somewhere around the age of fourteen they manifest as mutants. Deathlist, most feared hero killer in the world, started out as an ordinary schoolboy.
The Questport Chronicles: The Fellowship never finds out who the mage who destroyed Questport is or where he came from. No one had heard of him until after the cataclysm.
The heroine of Tired Fairy is originally just a joung woman from a middle-class family, going on university vacation. Then she finds the Sphere of Power, an ancient artifact that can kill averybody and destroy anything the wielder wishes (think Death Note on steroids). By the end of the first book, she is de-facto in control of the world.
In Worm, at the start of the story, Taylor is a bulled high school student with the weak power of controlling bugs. A few months later, she's the uncontested ruler of Brockton Bay's underworld, has fought off every hero in the city multiple times, defeated Dragon, and the heroes are terrified of facing her.
After two online users, ghettofinger and GirlVinyl were kept from creating a Wikipedia article about a certain someone, they went on and created their own wiki so they could get away with it. The result: The infamously offensive (and Nigh Invulnerable) Encyclopedia Dramatica.
You, in Cookie Clicker. At the very start, you can't even bake a decent cookie, with the ones you do make being unappealing even to the neighbourhood raccoon. Several gazillion cookies later, you've established a baking empire spanning several galaxies, have opened dozens of portals to universes made of cookies, baked with antimatter, unravelled time itself, and bribed elder gods, all with these same cookies that are now unimaginably good. And it all started with one click from someone who one day felt like baking some cookies.