coup de grâce
From Longman Dictionary of Contemporary Englishcoup de grâcecoup de grâce /ˌkuː də ˈɡrɑːs/ noun [singular] 1 DESTROYan action or event that ends or destroys something that has gradually been getting weaker2 KILLa hit or shot that kills someone or somethingOrigin coup de grâce (1600-1700) French “stroke of mercy”