House of the Scorpion: El Patron. He was born into a impoverished Mexican family, but at the time of the novel, he is one of the most powerful drug lords on the planet. Even after his death, his influence is still profound on the world.
Carrie. She goes from a quiet girl to a killer of many, all thanks to a mean-spirited prank. All together now: "They're all gonna laugh at you!"
Sandman Slim was just a magician who found magic easier than most. Then his buddies sent him to hell.
The Big Bad of Robin Jarvis' Deptford Mice trilogy is a terrifying and powerful being known as Jupiter. The prequel book The Alchymist's Cat reveals his beginnings. He was an ugly, runty kitten named Leech who was jealous of his strong and handsome brother, especially when he became a sorcerer's familiar. In the end, Leech killed his brother and took his power and identity. He crawled into a dark portal in the sewers and became a living god to the rats who dwelled there. They only ever saw his glowing red eyes and had no idea he was a cat.
In Outcast of Redwall we follow archvillain Swartt Sixclaw on his journey from leader of a small band of robbers to Warlord of a vast horde. Along the way he graduates from a psychopathic teenager to a brutal, charismatic general nicknamed "The Pitiless One."
In Jhegaala, it's mentioned that Easterners who became Teckla or Jhereg in the Dragaeran Empire, thereby gaining access to sorcery, and then returned to their homeland, often become this trope as soon as they realize they can still use a type of magic none of their countrymen have any defense against. It usually becomes necessary for the local rulers to intervene and put such upstarts down.
A series of science fiction books by Tony Daniel features Ames, an interplanetary dictator who sends virtual lifeforms to virtual deathcamps, and generally causes terror. He used to be a composer with an abusive childhood.
Gully Foyle from Alfred Bester's The Stars My Destination began as an uneducated Engineer's Mate 3rd Class aboard a freighter, and by the end he is a cybernetically enhanced unstoppable killing machine with the ability to teleport himself across the galaxy and through time.
Vito Corleone progresses by stages from a mild-mannered grocery clerk with no interest in anyone else's affairs to the head of the most powerful crime family in New York.
His son Michael Corleone goes from being a U.S. Marine with no interest or experience in the mafia, to assassinating the leaders of all four rival families, plus the ringleader of Las Vegas and his brother-in-law.
Pontius Glaw, recurring character and eventual Big Bad in the Warhammer 40,000Eisenhorn novels. Starting off as just another bored noble with a penchant for watching gladiatorial combat, a chance encounter with a pit fighter's Chaos-tainted piece of jewelery turned him into a powerful heretic and the head of a major Chaos cult. He was then knocked back to being a Nothing by getting himself mostly killed, eventually becoming nothing more than a personality engram encoded into a hunk of quartz. He then became a nightmare again, when Eisenhorn gave him a robot body in exchange for information; Glaw used the body to escape from his confinement, destroy Eisenhorn's carefully-constructed support network, and nearly unleash a powerful daemonic superweapon before finally being killed for good.
Gentleman Johnny Marcone began his criminal career as a two-bit nobody. When a little girl ended up comatose from a bullet meant for him, he took it upon himself to "clean up" Chicago's criminal underground. By the time of the books, he is established as one of, if not the most powerful (mortal) man in the city, and can casually deal with supernatural threats that even Harry has trouble with. He later signed on with the Unseelie Accords, a sort of supernatural UN, making him the first mortal to do so in it's long, long history. The worst of it is, Harry can't bring himself to do anything about him, because he tends to pull Harry's fat out of the fire. A lot. Although, there a few moments where he's less of a "Nightmare" as he is a "Mobster With A Heart." This being the other reason Harry is unwilling to harm Marcone.
Polonius Lartessa, one of the Denarian leaders is another example; when she started out she was just an underaged hooker in the temple of Isis. Then Nicodemus gave her the Denarian piece containing a fallen angel; since then she has ascended to a position where has not only become one of the most powerful mages on the planet, but has also caused numerous wars and ethnic cleansings (most notably the Rwanda Genocides and the Killing Fields of Pol Pot) purely for the hell of it. From nobody hooker to one of the biggest bastards in Literature.
The Archive. She's the magical receptacle for everything ever written down. Most Archives inherit this knowledge, the memories of past Archives at around age 30 or so, making them a straight example of this trope. The current Archive, though, has been the Archive since she was a few minutes old. She inherited it when her mother committed suicide, which Ivy remembers. In the Dresdenverse, knowledge is power. She can tango with the better part of two thirds of the strongest soldiers for Hell in the setting at one time, inside a trap made for her, with no preparation. She would have probably escaped, too, if not for Harry's hero complex and the fact that he's one of her only friends.
The current Summer Lady and Summer Knight. They were originally both changelings (one mortal parent, one Fae parent), who were a source of entertainment for the Winter Court (it's as unfortunate as it sounds). However, when Harry killed Aurora and freed the trapped Summer Knight (Lily), Lily became Lady Summer and appointed Fix as the Summer Knight.
Molly is this from the perspective of the White Council. A recovering warlock under the tutelage of another recovering warlock and one of the baddest Wardens of the White Council suddenly becomes a sovereign of the Winter Court who might have a significant grudge against the White Council. Oh and her mentor? Now ostensibly her Knight.
Wizards in general, but Harry especially. He starts out an orphan, the son of a poor, kindly stage magician. and Margaret LeFay McCoy, but hush. Now? He's taken on literal gods Winter and Summer Knights, Ladies, and Queens as well as several of their retainers, Greek godlings, ancient Native American nightmares, and so on, is on speaking terms with some such as Odin, plus some Angels, though they aren't technically gods, they're at the same unimaginable level of power, has killed at least a dozen of those gods: Lady Summer Aurora as well as the entire Red Court, he's currently the Winter Knight, has beaten to a standstill a creature that stops for nothing less than a nuke, and killed one of the heirs of Kemmler twice - Kemmler being a Warlock that the entire White Council and then some had to deal with, and who was responsible for WWI. One of those times, Harry was dead. He's ousted the White King, has killed uncountable vampires (Red, White, and Black), ghouls, and Fae, to the point of wiping out the entire Red Court, lead an assault on Arctis Tor, the center of the Winter Queen's power, resisted and converted the imprint of a Fallen Angel, established dominance over a creature so ancient it has achieved localized omniscience, works for Uriel, discovered the Black Council and killed at least several of its members], [[spoiler: used a loophole of the Laws of Magic to raise a ZOMBIE Tyrannosaurus Rex which won him a (temporary) reprieve after insulting the Erlking (the Summer King) by imprisoning him in a binding circle. He's also nearly killed 2000+ year-old Nicodemus Archleon, leader of the Knights of the Blackened Denarius TWICE with his bare hands, to the point that said villain is now terrified of Harry. Suffice to say, there is a reason Harry gets/needs reminded from time to time about why, despite doing heroic things, some of his acquaintances question how close he is to picking up the black hat, and why some of the "good guys" wonder if he hasn't already.
Darth Bane. From two-bit nobody miner on an Outer Rim world to one of the most powerful and evil Sith Lords of all time.
Arawn Death-lord from The Chronicles of Prydain began as nothing more then an ordinary mortal man. That is until the previous Death Lord Achren made him her consort and taught him Dark Magic. Arawn then betrayed Achren and seized the title of Death-Lord for himself
Rand al'Thor from The Wheel of Time went from a shepherd's son to being the single most powerful man on the face of the earth, able to destroy whole cities at will. Granted, he is The Chosen One and meant to save the world, but he is also prophesized to break the world too, and the fact he is quite rightly reputed to be a bit mad, and deteriorating, still makes a nightmare even to his allies.
From the same series, Padan Fain started out as a rank-and-file darkfriend whose cover was that he was a peddler. Then he got recruited by Ishamael to track Rand down, given minor magical abilities in the process. Then he merged with the soul of Mordeth acquiring a whole lot more power in the process and also going nuts. Blaming both sides of the battle of good against evil for his suffering, he's got it out for everybody- but especially Rand- and is unfortunately one of the more powerful beings in the setting now.
As lords go, Petyr Baelish is an example. He was born the son of the most minor lord in the Seven Kingdoms and raised among his social betters, earning him a massive inferiority complex. He spent his early adulthood using his financial genius to steadily raise his social station until earning the position of Master of Coin for the whole Seven Kingdoms. Since the start of the series, he's revealed himself to be one of the nation's most accomplished Chess Masters, with a seemingly limitless ambition, secretly instigating a devastating civil war that destroys entire families and tears Westeros apart, and using the chaos to become one of the most powerful men in the Seven Kingdoms.
Varys and Illyrio Mopatis, two Chess Masters, started life as an castrated orphan mummer boy and a homeless street bravo respectively. Through their partnership, however, they create a criminal empire that puts Illyio into a magister position in Pentos and Varys into a Master of Whispers position in Westeros. They use their positions to gain amazing wealth and influence.
And though he's already a lord, Walder Frey also counts, going from The Friend Nobody Likes old man who is openly spoken of with disdain by his allies to perpetrator of The Red Wedding and the killer of King Robb Stark.
Daenerys "Mother ofDragons" Targaryen seems to have been taking notes from her ancestor, "The Conqueror". From the penniless, youngest, Princess-in-Exile... to a Queen in her own right by dint of bloody conquest, forget rights from birth or marriage alone. Underestimating this escalating nightmare when you oppose her is not recommended.
Melisandre, born a slave, rises to become a formidable sorceress and one of Stannis Baratheon's two closest advisers (as well as being regarded as his 'true queen').
Doctor Impossible. From a nebbish perpetual undergrad mocked for his outdated theories to the third most dangerous man on Earth, who nearly destroyed the world with a toy, a mirror and a trinket.
General Woundwart from Watership Down was originally just an average ordinary rabbit who lost his parents from a young age who joined a gang of rabbit revolutionaries, then he became a power hungry dictator who enslaves rabbits and makes them work themselves to death and has his higher ranking officers do what they like to them including beating and occasionally raping them.
Claudandus aka Pascal from Felidae was an ordinary cat who was picked up off the streets for experimenting by scientists who vivisected him, along with many other cats, and being the only test subject to survive, this eventually drove him insane and killed the head scientist and became a serial killer of cats deeming them unworthy of resurrecting an ancient breed.
Katniss Everdeen from The Hunger Games. She was just another player, facing a one in twenty-four chance of staying alive longer than everyone else, but by the end of the series, she was the face of the revolution and played a role in the deaths of two corrupt Presidents of Panem.
In Wicked, there is the wizard who started as con artist and ended as an evil dictator of OZ, destroying everyone who opposed him.
The musical has him saying: "Hey look who's wonderful? This corn-fed hick.", referring to himself.
In Holes, Kissin' Kate Barlow, one of the most feared outlaws in the Old West was once a small-town schoolteacher. Then she fell in love with black onion seller Sam, and the townspeople burned down her school and lynched him.
Stag Preston of Spider Kiss went from a shady bellhop to a sociopathic superstar.
From Animorphs there's David. He is introduced as an ordinary kid with a BB gun and a pet cobra. Over the course of three books he goes from that to reluctant Sixth Ranger to vengeful Sixth Ranger Traitor who defeats the leader of the Animorphs in single combat and comes closer to wiping out the team than the entirety of an alien empire.
Goose Bumps: The people of Dark Falls in Welcome To Dead House were a relatively humble community until toxic gas escaped from the local plastics factory. This mutated every man, woman, and child into an undead monster, and now they expand their numbers every year by feeding on visitors.
Della Raver from The Curse Of Camp Cold Lake was an average girl who hated her life at camp. She was killed trying to escape, and her spirit became the skeletal nightmare we see on the cover.
The main villain of The Horror At Camp Jellyjam was once a snail whose underground cavern was polluted by jello from the above camp, and mysterious radiation. Contacting both substances, he was mutated, growing in both size and intelligence. Fearing that his stench would destroy him,the creature found he could telepathically influence others when councillors answered his cries and tended to him. Eventually, the beast came to relish the power he had over human lives, and crowned himself King Jellyjam, one of the most bloodthirsty and horrifying monsters in the Goosebumps canon.
Harry Potter, Lord Voldemort went from living in a Muggle orphanage as a child to being the Big Bad as an adult. And got defeated by a boy who spent most of his early years living in a closet under the stairs of people who don't like him.
Kara no Kyoukai: Lio Shirazumi used to be a timid high school kid with a crush. He wanted to impress his crush by getting in a fight, but things got out of hand and he needed to find a way to dispose of a body. Then he eventually grew to become a cannibal with a crush...
Just Another Judgement Day, the ninth book of the Nightside series, introduces The Walking Man. He was just an ordinary human being, until he was given a mission from God to purge all the evil in the Nightside—and enough Power to do so with extreme prejudice. The Walking Man shrugs off any and all attempts to hurt or kill him, or otherwise stop his mission. No one and nothing in the Nightside is effective against him. John Taylor is only able to "defeat" him by finding the vanilla mortal he used to be and separating him from his power.
Robert Bloch's "Sweets to the Sweet" has Irma, who was unfortunate enough to have her mother suffer Death by Childbirth. Blaming her for this, her father regularly beats and abuses her, constantly calling her a witch. Her uncle is no better; though he's fully aware of the abuse, he ignores it and goes about his own life, and gets irritated when the housekeeper begs him to intervene. So she resolves to become exactly what her father thinks she is, and studies witchcraft, creating a tiny wax Voodoo Doll of her father. And ultimately bites its head off, then slips away while her uncle rushes to her father's bedside.
Not exactly evil, but Dr. Alexandre Manette ends up nearly being the death of his son-in-law Charles Darnay in Dickens' A Tale of Two Cities. In one of those trumped-up coincidences Dickens was so famous for, Dr. Manette is imprisoned in the Bastille for many years thanks to the Darnays' influence and writes an angry journal entry condemning the whole family. His daughter Lucie later marries Charles, the young son of the family who had imprisoned him. When the French Revolution comes, a murderous people's tribunal puts Charles on trial and submits as evidence Dr. Manette's long-forgotten denunciation of his family. Charles is promptly sentenced to death and is due to be beheaded by the guillotine in 24 hours. If not for a certain Heroic Sacrifice....
For that matter, Ignoring concepts of predetermination, Hari/Caine himself. Hari was just some street urchin with a crazy father who became the greatest actor Earth ever knew. And Caine was just some assassin who became the scariest bastard Home ever knew (who directly started at least four wars).
Erik was born in a small town not far from Rouen. He was the son of a master-mason. He ran away at an early age from his father's house, where his ugliness was a subject of horror and terror to his parents. For a time, he frequented the fairs, where a showman exhibited him as the "living corpse." He seems to have crossed the whole of Europe, from fair to fair, and to have completed his strange education as an artist and magician at the very fountain-head of art and magic, among the Gipsies. A period of Erik's life remained quite obscure. He was seen at the fair of Nijni-Novgorod, where he displayed himself in all his hideous glory. He already sang as nobody on this earth had ever sung before; he practised ventriloquism and gave displays of legerdemain so extraordinary that the caravans returning to Asia talked about it during the whole length of their journey. In this way, his reputation penetrated the walls of the palace at Mazenderan, where the little sultana, the favorite of the Shah-in-Shah, was boring herself to death. A dealer in furs, returning to Samarkand from Nijni-Novgorod, told of the marvels which he had seen performed in Erik's tent. The trader was summoned to the palace and the daroga of Mazenderan was told to question him. Next the daroga was instructed to go and find Erik. He brought him to Persia, where for some months Erik's will was law. He was guilty of not a few horrors, for he seemed not to know the difference between good and evil. He took part calmly in a number of political assassinations; and he turned his diabolical inventive powers against the Emir of Afghanistan, who was at war with the Persian empire. The Shah took a liking to him.
In the Age of Fire series, the Copper was ostracized from his family at early age and was considered a cripple and misfit by the Lavadome Dragons. By the end of his book, he's their leader, and has his sights set on conquering the world.
The villain from The Moment Of The Magician was a balding, overweight loser who'd scraped by doing cheap tricks for kids' birthday parties, until he found himself in a world where his magic worked.
Rattila, the Ratislavan villain from Myth-Taken Identity, was a janitor at a magical research lab until he stole an experimental artifact that vastly increased his power.
Timeline-191: Jake Featherston was an artillery sergeant with a bad temper, before he ran into the Freedom Party. His rage at losing the Great War and skill as an orator has him become the thirteenthPresident of the Confederate States of America following the Great Depression. He than starts the Second Great War, whilst conducting a genocide of the CSA's black population. Most of his major henchmen, including Jefferson Pinkard (steel worker and veteran turned concentration camp commander) and Clarence Potter (intelligence major turned Brigadier-General, Spymaster, and all around Magnificent Bastard) also count.
Norman Arminger of the Emberverse was an unknown professor of history whose hobby was mock-swordfighting in the SCA. When the Change put paid to guns and most modern technology, he swiftly rose to become "Lord Protector" of the Portland Protective Association; that is, a brutal despot running what amounted to a protection racket.
Raistlin the Wizard from the Dragonlance series barely survived the test to become a wizard (you fail, you die), and even then only because he was allowed out of pity to have his warrior brother help him. He went from almost being killed from touching a Dragon Orb to being able to wield an Orb's considerable power without even looking at it, and eventually his power rivaled that of the Dark Queen, the Dragonlance universe's equivalent of Satan. In keeping with this trope, he lost his humanity as his power increased.
Otha from The Elenium. Originally a shepherd boy with a penchant for hurting animals, he stumbles across an idol in a field and eventually becomes a 1,900 year old emperor with vast magical powers and one of the most powerful armies in the world at this disposal.
There's Mr. Teatime from Hogfather, a quiet, generally soft-spoken scholarship boy who is contracted to assassinate (the scholarship was to the Assassin's Guild school) a mythological creature, the Discworld's equivalent of Santa Claus. And damn near succeeds.
Gretchen Richter in 1632. She was first a printer's daughter, then she was reduced to the concubine of a mercenary captain. As of now, she's the leader of a powerful, well-armed political movement spanning half of Europe, and is said to figure prominently in the nightmares of most of Europe's nobility. During the civil war in 1636: The Saxon Uprising, she took over Dresden, and held it for weeks against the Swedish General Baner, until the Third Division (including her husband) showed up and smashed Baner's army.
In The Dog Stars, Bangley has fought off countless bandit invasions After the End over the course of nine years. He keeps his backstory mysterious, but eventually Higs learns that he was just some farmer who was abused as a kid. He's been itching for a time when he could do nothing but kill people, and it turns out he's incredibly good at it.
In Sukhinov's book Gingema's Daughter (Continuation of Wizard of Oz) there is Corina. She starts as an ordinary Munchkin girl who didn't want to spend her entire life collecting herbs so she ran away from home. She gets lessons from Gingema ( aka The Wicked Witch of the East), and from then on she gradually gets more and more powerful. By the end of the book, she is the most powerful witch in OZ, rules the City of Oz, has The Woodsman (who rules the Violet kingdom) under Mind Control, and effectively rules the rest of Magical Land too (with the exception of Glinda's and Wiilina's kingdoms). And she has ''Dragons'' on her side.
In Seven Sorcerers, Arafin Strood was originally an ordinary human, servant of a renown alchemist Gan Mafig. The the titular sorcerers tested the Deathbane on him, which gave him Complete Immortality - and tested it by repeatedly tearing him to shreds. Now he is the most horrible villain the the magical realm.
Geder Palliako from The Dagger and the Coin is introduced as the nerdy, overweight son of a minor nobleman and isn't taken seriously by anyone, even himself. Then he gets put in charge of an occupying force thanks to political wrangling and proves himself to have a disturbingly sociopathic streak that manifests when he both has power and feels threatened. Then he meets Basrahip, the leader of a Path of Inspiration who decides Geder is The Chosen One- and has a Compelling Voice that lets him make other people believe it to. Things end up with Geder as Evil Overlord of half the continent, and still rather stunned at the chain of improbable events that got him there.
Both Ian Covey and David Tirado spend Everyman changing from normal men into literal monsters.
Honor Harrington starts out as a gifted but otherwise rather unimportant Commander in the Royal Manticoran Navy. Then she's sent to Basilisk Station. Twenty years later, she is known as "The Salamander" for always being where the fires are hottest, is considered her generation's answer to the military legends Edward Saganami and Ellen D'Orville, is on first-name terms with the rulers of three separate star nations plus a good chunk of their governments, and has played a critical role in forging an alliance between her star nation and the one she's spent most of her career fighting. Not to mention collected a couple of noble titles and a knighthood.
Second Apocalypse: Kellhus is just a wanderer who is considered notable only because he happens to hang around Cnaiur. Very quickly, however, he has set himself up as a prophet and seized control of the entire Holy War. It only goes up for him from there.
The alternate history novel For Want of a Nail gives us Bernard Kramer, a German immigrant in the United States of Mexico. Spending years trying to scrape out a living as a miner, he eventually puts together enough capital to found future Mega Corp. Kramer Associates. Through this company, he would then go on to gain enough political power to do things like fund a pro-Mexican revolution in Guatemala for the sake of his business interests, and gain enough influence to effectively control the Mexican government up to and including installing a dictator.
Kramer Associates as a whole probably counts. Starting as a 26 member shipping and dry goods company, it would, as mentioned above, go on to become a Mega Corp. of such influence that it practically controls politics in the Western hemisphere, becoming a nation unto itself, even developing nuclear weapons before any of the world's actual nations.
Monsieur Thenardier went from a crooked innkeeper and a petty criminal to the leader of a feared crime gang. By the end of the book he's a rich slave driver in America.
There was once an ordinary woman named Jennet Humfrye. She bore a son outside of wedlock that she was unable to provide for and the pressure of her family forced her to give her child to her rich, childless sister to adopt. But she wanted her son to be hers and only hers and tried to spend more time with him. Like all mothers. Then a horrible tragedy happened and the son drowned in a accident in front of her eyes. She died not long after, in misery and bitterness, unfairly blaming her sister for the death and cursing all mothers who never had their children taken from them like she did. And then this woman became a shadow of death and one of the biggest horrors knows as The Woman in Black.
Seerdomin manages this twice: Once the son of a fisherman, he became a high ranking officer in an army full of religious fanatics and cannibals. Then, after that career path has endet, he does it again in Toll the Hounds, when he goes from gloomy but generally decent citizen to stalking and murdering people at night because they were planning to get rid of their new lord, whom he secretly approves of.
Before Shadowthrone ascended to be the new Lord of Shadow, he started off as a bar owner and middling criminal. Then he took some levels in badass when he planned the conception of the the continent-spanning Malazan Empire as Emperor Kellanved. Well, and then there is the whole 'new god' thing.
Thomas Cromwell in Wolf Hall, once he begins to implement Henry's more brutal policies. He was born the son of a blacksmith in Putney (a past he tries to leave behind and which his highborn adversaries remind him of whenever he irritates them), but his job as Wolsey's lawyer leads him to become Henry VIII's right-hand man, and he readily takes advantage of his position to exact deadly revenge on those who caused Wolsey's downfall.
The nameless protagonist of Ratman's Notebooks was a humdrum, put-upon loser, nagged by his mother and intimidated by his boss. Then he opted not to kill the family of rats living in his garden. Within months, he's robbing houses, terrorizing the neighborhood in a creepy rat mask, and siccing an attack-trained Squeaking Carpet on his personal enemies.
In the Queen and Country novel A Gentleman's Game, we're introduced to William Leacock. Starts off as a harmless British kid, and ends up becoming a bloodthirsty jihadi who murders his best friend, a girl he's fallen in love with, and Tom Wallace, the love interest of series main character Tara Chace.
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.
In general this can happen a lot due to the mechanism of Super Empowering in the setting: A number of people who hit the Despair Event Horizon, not exactly a difficult task in this Crapsack World, undergo a Traumatic Superpower Awakening called a "Trigger" and start manifesting superpowers. Anyone could do so and none of the precognitives in the setting have been able to predict who will manifest or what they'll get despite decades of research of varying ethicality.