sword of Damocles

From the following story: Damocles was an obsequious courtier in the court of Dionysius II of Syracuse, a fourth century BC tyrant of Syracuse. Damocles exclaimed that, as a great man of power and authority, Dionysius was truly fortunate. Dionysius offered to switch places with him for a day, so he could taste that fortune first-hand. In the evening a banquet was held, where Damocles very much enjoyed being waited upon like a king. Only at the end of the meal did he look up and notice a sharpened sword hanging directly above his head, held only by a single horse-hair. Immediately, he lost all taste for the festivities and asked leave of the tyrant, saying he no longer wanted to be so fortunate. Dionysius had successfully conveyed a sense of the constant threat under which a powerful man lives. From Ancient Greek Δαμοκλῆς (Damoklês).

来源：wiktionary