the.com/swan song

means A person's last act, performance, or work before retiring, departing, or dying — often treated as a fitting, deliberate farewell.

from Rests on an ancient belief, found in Greek writers like Aeschylus and Plato, that the mute swan sings one beautiful song only as it dies. Biologically it's false — swans don't croon their way out — but the legend was too good to let go, and by the early 1800s English had borrowed the image whole as 'swan song' for any final, crowning gesture before silence.