death knell

from the GNU version of the Collaborative International Dictionary of English.

• noun A stroke or tolling of a bell, announcing a death; a knell{1}.
• noun A sign or harbinger of the end, death, or passing away of anything.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun The tolling of a bell announcing death.
• noun idiomatic, by extension A sign or omen foretelling the death or destruction of something.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun an omen of death or destruction
• noun a bell rung to announce a death