I'm searching for a word, phrase or term that means the fatal blow, something along the lines of coup de grace. However, that term has implies a merciful killing. I'm looking for something that connotes a savagery; a complete evisceration.
For example, you're describing the arguments an attorney is making to a court. The attorney is hammering the adversary. Then s/he makes this [coup de grace] argument to just drive it home. There's nothing merciful about it (that's why I don't like coup de grace in this context).