In the myth of Orpheus and Eurydice, Orpheus looked back and caused her return to the Underworld.
Orpheus was a legendary musician whose singing and lyre playing could charm animals, plants, and even the inhabitants of the Underworld. After Eurydice died from a snakebite, he descended into the realm of Hades to ask for her release.
Hades agreed on one condition: Orpheus had to walk ahead of Eurydice and not look back until both had reached the upper world. Near the end of the journey, Orpheus turned to see whether she was following him. Because he had broken the condition, Eurydice disappeared back into the Underworld.
The story appears in Greek and Roman literary traditions, including works by Virgil and Ovid. Different versions vary in details, but the backward glance is the decisive event in the familiar narrative.