Gun.
Weapons that use the pressure of gunpowder combustion to eject bullets.
It was a weapon of mass destruction.
It completely changed the pattern of the war and made killing more efficient. It ended the battle between the spear and shield that had lasted for thousands of years, causing humans to completely transcend the food chain and stand above all creatures, except cats.
Be it ancient, modern, extinct, or still present, the so-called ferocious predators were only a pile of rotten meat under a large-caliber rifle. Even if the Tyrant Dragon came, it was useless. This was the sorrow of carbon-based creatures. Protein was extremely fragile to begin with.
Although it was very powerful, the principle was very simple. Even children knew.
The basic principles of truly powerful things were often very simple, just like how modern humans only had two sources of strength—to cause explosions and boil water.