The An Lushan Rebellion
At the height of the brilliant Tang dynasty — China's cosmopolitan golden age of poetry and trade — an over-mighty frontier general named An Lushan turned his armies on the capital, igniting a civil war that broke the dynasty's power for good. The chaos and census collapse that followed were so vast that some counts rank it among the deadliest events in all of history.
A sudden collapse in population (plague, famine, mass violence) creates labor scarcity that reorders economy and society.