Source : Webster's Revised Unabridged Dictionary (1913)

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

         The deathblow of my hope.                --Byron.

Source : WordNet®

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