"The Sword of Damocles" is a literary phrase often used to denote imminent danger. Its origin is a tale from ancient Greece.

Damocles was an active and well-known courtier during the reign of Dionysius the Elder, around 400 BC. At one point, he went overboard in his flattery of the Greek ruler's privileges of power. As a reward, Dionysius invited him to a banquet to show appreciation for the praise. Arriving at the feast, Damocles saw that there was a sword hanging over his chair, suspended by a single horse hair. As refusing to sit would have been a mortal insult, he was forced to take his perilous seat. Naturally, his enjoyment of the meal was hampered by his awareness of his peril, and the subtle message that Dionysius was sending was evident: A ruler's position is not all feasts and indulgence; his responsibilities hang heavy over him.

Incidentally, this is also where the phrase hanging by a thread comes from.