樂昌破鏡

lit. the story of the broken mirror of Princess Lechang 樂昌公主|乐昌公主[Le4 chang1 Gong1 zhu3] (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