death knell

1. A stroke or tolling of a bell, announcing a death; a knell{1}.

2. Hence: (figuratively) A sign or harbinger of the end, death, or passing away of anything.
[PJC]

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.

The noun has 2 meanings:

Meaning #1: an omen of death or destruction

Meaning #2: a bell rung to announce a death
  Synonym: death bell