Fan Casting From Nobody to Nightmare in Sort by Tropes

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 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.