Im sorry Naomasa-dono, i downloaded my copy from napster and im not sure if it is truly in the soundtrack, but i believe it is, the title i believe is "Bushido" by Yo Yo Ma, and that is the same key words i used to search for the song, truly haunting indeed.

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In my sword; the wind, in my heart; courage, in my eyes; death...I am Minagawa