Orpheus and Eurydice
Orpheus was betrothed to the Nymph Eurydice, with whom he was deeply in love.
One day while she was walking along the bank of a river, she encountered the shepherd Aristaeus. Captivated by her beauty, Aristaeus fell in love with her and pursued her across the fields.
Eurydice tried to escape, but as she ran she stumbled upon a serpent, which bit her with its lethal venom. Devastated by his loss, Orpheus decided to travel to the underworld (from which no mortal had ever returned), in order to have his wife restored to him.
Persephone (Proserpina), queen of the underworld, was so moved by his grief that she agreed to grant his wish on the condition that he not look back at Eurydice on the journey back to the light. But as the end of his journey drew near, Orpheus could not help looking back to make sure his beloved was still beside him. The moment he looked at her, she vanished before his eyes and he lost her forever. Orpheus never recovered and lived with that suffering for the rest of his days.