Knell is almost exclusively a word of death — so specialized that "death knell" is technically redundant, yet we say it anyway because some endings demand the emphasis.
The sound of a bell, especially when rung solemnly for a death or funeral; a signal of the end or failure of something.
From Old English cnyll (a sound of a bell), related to cnyllan (to ring a bell, to toll), from Proto-Germanic *knulljaną (to strike, to sound). The word has maintained its specific association with death and mourning throughout its history. Key roots: *knulljaną (Proto-Germanic: "to strike, to sound, to knock").
The knell is one of English's most evocative words — it exists almost exclusively in the context of death and ending. John Donne wrote "never send to know for whom the bell tolls; it tolls for thee," and Keats opened a famous ode with "My heart aches, and a drowsy numbness pains / My sense, as though of hemlock I had drunk" before invoking the nightingale's song as a kind of knell. The phrase "death knell" is almost redundant, since knell already implies death — but English