sword of Damocles

Also, Damocles' sword. Impending disaster, as in The likelihood of lay-offs has been a sword of Damocles over the department for months. This expression alludes to the legend of Damocles, a servile courtier to King Dionysius I of Syracuse. The king, weary of Damocles' obsequious flattery, invited him to a banquet and seated him under a sword hung by a single hair, so as to point out to him the precariousness of his position. The idiom was first recorded in 1747. The same story gave rise to the expression hang by a thread.

An imminent and/or constant threat. According to Greek legend, Damocles, a friend of King Dionysius of Syracuse, envied the ruler's great wealth and power. When Damocles told the king how fortunate he was, Dionysius offered to change places for a day. As Damocles dined at the head of the table, he happened to look up. There above his head, held by only a single horsehair, hung a sharp sword pointing downward toward his chair. Frozen with fear that the thread would break, he pointed out the predicament to the king. Dionysius nodded, acknowledging that the sword was a constant factor in his life, an actual and a metaphoric reminder that some person or circumstance might at any time cut the thread. Such risk, the king added, comes as an integral part of power. Any ever-present risk, especially one that's hanging by a thread, is how the phrase has been used.