deal a mortal blow to (someone)

1. To wound someone in a way that leads to their death. The knight dealt a mortal blow to his enemy with a single thrust of his halberd. He's so cocky in battle that I know someone will deal a mortal blow to him sooner than later. The warrior swiftly dealt a mortal blow to his adversary on the battlefield.

2. By extension, to do something that ensures someone's or some group's ruin, failure, or defeat. The scandal over tax evasion has no doubt dealt a mortal blow to the senator's hopes for reelection. These onerous new federal regulations are going to deal a mortal blow to our entire industry. I'm worried that this massive recall will deal a mortal blow to our company.