Hades summoned Eurydice's spirit and told Orpheus that she would follow
up to the world of day and be his living wife again under one condition---- if he looked back at her before they stood in the sunlight together, she would vanish
back into the underworld and no new pleas would convince Hades to release her again. Hades set this condition to measure Orpheus if he could keep his promise but, Orpheus was not able to keep his promise.
