The most terrifying thing in the world - a teenage girl!

Explanation: During the Hundred Years War, England and France fought over the ‘true’ heir to the French crown. France, by the 1420s, wasn’t doing so well, and it looked like England might win the war decisively.

However, a French peasant girl known as Joan of Arc suddenly claimed to have heard the voice of God and His angels, telling her to drive the English out of France. She was actually very interested in military matters from a young age, to the point where her father threatened to drown her if she ever tried to run off to pursue such matters (which would have been unacceptable for her to participate in under normal circumstances, considering her gender and class).

Nonetheless, her religious zeal and conviction gave her widespread support at a time of crisis for France, and she led France to several important victories over the English, turning the tide of the war. While she would be captured and burned at the stake by the English for the crime of wearing men’s clothing, her contributions to the French war effort would lead to France eventually winning the war.