The Collaborative International Dictionary of English v.0.48:

Deathblow \Death"blow`\, n.
   A mortal or crushing blow; a stroke or event which kills or
   destroys.
   [1913 Webster]

         The deathblow of my hope.                --Byron.
   [1913 Webster]

WordNet (r) 3.0 (2006):

deathblow
    n 1: the blow that kills (usually mercifully) [syn: deathblow,
         coup de grace]

Moby Thesaurus II by Grady Ward, 1.0:

62 Moby Thesaurus words for "deathblow":
   KO, Waterloo, bane, beating, clincher, collapse, conquering,
   conquest, crash, crusher, death, death knell, death stroke,
   debacle, defeat, destruction, doom, downfall, drubbing,
   early death, early grave, end, end-all, ender, failure, fall, fate,
   final stroke, finisher, finishing stroke, hiding, kayo, kayo punch,
   knockout, knockout blow, lambasting, last dab, lathering,
   lethal blow, licking, mastery, overcoming, overthrow, overturn,
   quietus, ruin, settler, smash, sockdolager, stopper,
   stroke of death, subdual, subduing, subjugation, sudden death,
   thrashing, trimming, trouncing, undoing, untimely end,
   vanquishment, whipping