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.