Fig.: the last appearance, performance or work of an artist such as an actor, a musician or a writer; one's last words of actions. By extension, it came to indicate someone's finest work, the culmination of all his/her artistry

It looks like the great champion's career is on the wane. Last year's world title has been his swan song

Probabilmente anche per il grande campione è cominciato il declino. Il suo canto del cigno è stata la vittoria ai mondiali dell'anno scorso

The last project he worked on before retiring was the best that he had ever done: it was truly his swan song

L'ultimo progetto su cui ha lavorato prima di andare in pensione è di gran lunga il migliore che abbia mai fatto ed è considerato il suo canto del cigno

According to Plato (Phaedo, 84e-85b), the swan, that couldn't sing like all the other birds, would sing a glorious song when it felt the approach of death. This because the swan, that was sacred to Apollo – god of music and poetry –, knew that it shortly was going to join the god it served.
“… the swans …, which when they sense that it necessary that they die, they sing in the interval before death, indeed, at that time, especially and most beautifully do they sing, rejoicing that they are about to go to the divine, the very thing they serve.”