一鸣惊人

idiom

to amaze the world with a single brilliant feat; to achieve overnight success; to shock everyone with one's first attempt

Originally from the 'Records of the Grand Historian' (《史记•滑稽列传》), describing a bird that does not sing until it sings in a way that startles everyone. Used to describe someone who usually keeps a low profile but achieves something extraordinary when given the opportunity. Often used to describe debut performances, first-time successes, or hidden talents suddenly revealed.