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 timelines don'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. In scenarios where the villain rose by their own effort, this often a case of I Did What I Had to Do, Jumping Off the Slippery Slope, and maybe a few Angst Nukes thrown in for good measure. This may owe itself to the shock value of the idea that anyone and anything can become the Big Bad under the right circumstances; 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?". (Or becoming He Who Fights Monsters.) Frequently, the details of this process are uncovered in the villain's Start of Darkness. Contrast From Zero to Hero. Also see Who's Laughing Now?, Malignant Plot Tumor, and Magikarp Power.
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. In such a case, the inverse trope is Villain Decay.
