乐昌破镜

lit. the story of the broken mirror of Princess Lechang 乐昌公主 (Lè chāng Gōng zhǔ) (In the tale, the princess and her husband, fearing separation during the turbulence of war, broke a bronze mirror in half. They each kept one half as a token, with the promise to reunite by matching the pieces together. They were indeed separated, but eventually reunited, with the mirror playing a crucial role in their reunion.) (idiom); fig. the reunion of separated lovers or the restoration of a relationship