This line alludes to the Greek myth of Orpheus. In the story, Orpheus travels to the underworld to rescue his lover Eurydice, who was bitten by a viper on their wedding day. Orpheus plays music so beautifully for Persephone and Hades that they allow him to take Eurydice back to the world of the living under one condition: he cannot look at her until they are out of the land of the dead. Orpheus leads his love to the surface and resists looking back at her until he is fully back in the sunlight. Tragically, Eurydice is still on the threshold of the underworld when Orpheus turns and looks at her. She vanishes instantly back to the underworld, forever out of reach from her living lover.