Sword of Damocles: A metaphor based on Greek legend of the constant fear present in the lives of those with great responsibility. Damocles was a courtier for King Dionysis. After constantly praising Dionysis’ great fortune, Damocles was offered the chance to switch roles with Dionysis. When Damocles became king a sword appeared over his head suspended by only a single horse hair. The fear that the sword would fall proved too much for Damocles. He begged Dionysis to let him return to his role as a courtier.

Source: Wikipedia