Complex sentence: It opened the theatre's Carnival season on December 26,1813 with the famed castrato, Giovanni Battista Velluti as Arsace--the only role that Rossini wrote for the castrato voice.
Split into simpler sentences: It opened the theatre's Carnival season with the famed castrato, Giovanni Battista Velluti as Arsace. It was the only role that Rossini wrote for the castrato voice.