death knell

noun2

1. •
   The tolling of a bell announcing death.

   • The sound was the stroke of a bell from the tower of East Endelstow Church. […] The death-knell of an inhabitant of the eastern parish was being tolled. — 1872
2. •
   A marker, sign or omen foretelling the imminent end, death or destruction of something.

   • The thought was a death-knell to Helen's last hope, and she sank down, quite overcome;[…] — 1901