FTP, name this opera set in Boston and composed by Giuseppe Verdi.
The main theme of this opera's orchestral prelude is the hero's first-act love song, "La rivedro."  It is during an attack by Samuel and Tommaso that the central deception is revealed.  This happens after a series of revelations at the hut of the suspected witch Ulrica.  Finally, Renato discovers that his wife, Amelia, loves his best friend, Ricardo, and hatches a murderous plot.  Its setting was shifted from the original locale of Sweden during the period of Gustavus III's assassination at the title event.  FTP, name this opera set in Boston and composed by Giuseppe Verdi.
The main theme of this opera's orchestral prelude is the hero's first-act love song, "La rivedro."  
A Masked Ball or Un ballo in maschera