Final answer:
The longbow was the Medieval invention that led to the downfall of knights as the most successful warriors in battles like the Battle of Crecy and the Battle of Agincourt in the Hundred Years' War.
Step-by-step explanation:
The Medieval invention that led to the downfall of the knight as the most successful warrior in battles like the Battle of Crecy and the Battle of Agincourt in the Hundred Years' War was the longbow.
The longbow, used by the English, was prized for its ability to send arrows farther and faster than the French crossbow. Its effectiveness rendered the heavily armored knights vulnerable, as they could be easily killed from a distance by the longbow arrows.
The introduction of the longbow changed the nature of warfare, minimizing the role of knights and shifting the focus towards large professional standing armies paid for by monarchs.