deathblow

noun2

1. •
   A strike or blow that leads to death, especially a coup de grâce.

   • He was also there at that fight when Hygelac folded under his deathblow. — 1961
2. •
   (figurative) Something that prevents the completion, or ends the existence, of a project etc.; a fatal setback.

   • The opening of the bridge dealt a deathblow to the ferry.