The Scythians
The Scythians were the first steppe nomads to master mounted archery and terrify the settled world with it — shooting accurately from a galloping horse, refusing pitched battle, and vanishing into grassland. Persia's king Darius invaded to punish them and found nothing to punish: they retreated, burned the grass, and let his army starve. They had no cities to take. Their goldwork, buried in mounds, is extraordinary, and their tactics established a template that Huns, Mongols, and Turks would each rediscover.
A weapon or tactic upends the prevailing balance of power and renders an old defensive or offensive order obsolete.