verb

• (intransitive) To ring a bell slowly, especially for a funeral; to toll.
  "I’ll make thee sick at heart, before I leave thee, And groan, and die indeed, and be worth nothing, Not worth a blessing nor a bell to knell for thee […]"
• (transitive) To signal or proclaim something (especially a death) by ringing a bell.
  "Let thy friends be as the dead in doom, And build to them a final tomb; Let the starred shade that nightly falls Still celebrate their funerals, And the bell of beetle and of bee Knell their melodious memory."
• (transitive) To summon by, or as if by, ringing a bell.

noun

• (figuratively) A sign of the end or demise of something or someone.
  "But at the close of the war there was less thought of what [Britain] had retained than of what she had lost. She was parted from her American Colonies; and at the moment such a parting seemed to be the knell of her greatness."