Sword of Damocles, a constant and immanent peril; a popular metaphor for any great and threatening evil that may befall one at any time.
Example usage: “the possibility hangs over their heads like the sword of Damocles”
Origin: it comes from Roman times, where an individual was seated at a banquet with a sword suspended over his head by a single hair to to remind him of the precariousness of the power and privilege which he envied.
Sources:
http://users.tinyonline.co.uk/gswithenbank/sayingss.htm
http://wordnet.princeton.edu/perl/webwn?s=sword%20of%20damocles
http://dictionary.reference.com/browse/Sword%20of%20Damocles
