The Discovery of Gunpowder
Chinese alchemists searching for an elixir of immortality instead stumbled on its opposite: gunpowder, a mixture that would go on to kill more people than any substance in history. For centuries the Chinese used it mainly for fireworks and simple bombs, but the recipe crept along the trade routes, and in European and Ottoman hands it became the cannon and the gun. Gunpowder would eventually shatter the medieval order of knights and castle walls — the same kind of rupture the atom bomb would one day be.
A weapon or tactic upends the prevailing balance of power and renders an old defensive or offensive order obsolete.