meet one's Waterloo

To encounter one's ultimate obstacle and to be defeated by it: “After beating dozens of challengers, the champion finally met his Waterloo.” From the Battle of Waterloo, where Napoleon Bonaparte was finally defeated.

Suffer a major defeat, as in Our team's done well this season but is about to meet its Waterloo. This term alludes to Napoleon's defeat at Waterloo, Belgium, in 1815, marking the end of his military domination of Europe. It was being transferred to other kinds of defeat by the mid-1800s.