WordNet (r) 3.0 (2006):

sword of Damocles
    n 1: a constant and imminent peril; "the possibility hangs over
         their heads like the sword of Damocles"

Moby Thesaurus II by Grady Ward, 1.0:

42 Moby Thesaurus words for "sword of Damocles":
   bulldozing, chanciness, commination, criticalness, delicacy,
   denunciation, doubtfulness, dubiousness, empty threat, foreboding,
   hazardousness, idle threat, imminence, implied threat, insecurity,
   instability, intimidation, menace, perilousness, precariousness,
   promise of harm, riskiness, shakiness, slipperiness, threat,
   threateningness, threatfulness, ticklish business, ticklishness,
   totteriness, touchiness, uncertainty, undependability,
   unhealthiness, unpredictability, unreliability, unsafeness,
   unsoundness, unsteadiness, unsureness, untrustworthiness,
   warning