death knell
A signal of the impending end or ruin of something, especially a business, organization, or activity. Refers to the sound of a bell ringing (the knell) from a church to indicate that someone has died. The geometric expansion of the Internet and digital media has tolled the death knell for countless print-based businesses around the world.
death knell
Something that indicates impending failure, as in His low scores sounded the death knell for his ambitions. The noun knell, used for the ringing of a bell since at least a.d. 1000, is rarely heard today except in this figurative phrase.