Meet One’s Waterloo
Etymology
From the Battle of Waterloo in 1815, where Napoleon Bonaparte, returned from exile, was decisively beaten by the allied forces of Wellington and Blücher — ending his rule for good. Because it terminated a career of near-continuous conquest, "meeting one’s Waterloo" came to mean the defeat that finishes a winning run. It marks not just any loss but the one that ends the story.
verb · /MEET wuhnz WAW-ter-loo/
To suffer a final, decisive defeat after a run of success.
What it names
The phrase for the conclusive loss that ends an otherwise dominant career or streak.
Example
The undefeated champion finally met her Waterloo against a rookie half her age.
Related idea
A Pyrrhic victory is a win so costly it ruins you; meeting one’s Waterloo is a flat, decisive defeat — a hollow triumph versus a conclusive loss.