Written when was 30, and thus nearing the halfway mark of his productive career, Sibelius's The Swan of Tuonela is one movement from his Lemminkainen Suite, a collection of four tone poems that tell the story of one of the heroes of The Kalevala, the national epic of Findland. Lemminkainen undergoes a number of tests and trials, one of which is that he is tasked with killing the Swan of Tuonela. Tuonela is the land of the dead, and the swan is a mystical beast living in Tuonela's waters. Lemminkainen tries to kill the swan, but is himself shot with a poisoned arrow and dies. He is apparently restored to life later in the story, but this particular movement is primarily concerned with the swan itself.
This work is meditative and almost without bombast of any kind, instead engaging a kind of brooding mysticism that reminds me of Rachmaninov's Isle of the Dead. A solo English horn portrays our swan, moving with churning placidity through its mournful waters, as the orchestra sings plaintively along with it. It's a remarkable work.