If you want a classical piece, why not choose an aria from one of the tragic operas? For instance, in Act IV, Scene 2 of Giuseppe Verdi's Aida, Radames has been thrust into the crypt in the Temple of Vulcan and awaits death. As he does so, he utters a quiet wish that Aida may be happy wherever she is, and never learn of his dreadful end. Soon, however, the figure of Aida is seen approaching and has been waiting for Radames for three days. Radames attempts to lift the stone and free her are futile.
Aida sings her last farewell to earth (O terra, addio), and Radames joins in with her. A poignant duet, indeed.