Complex sentence: He chose cello as the main instrument for the music as he thought it has a "darker sound" that suited the show, on which a violin would later joined that may suggests an interplay between different characters.
Split into simpler sentences: He chose cello as the main instrument for the music as he thought it has a "darker sound" that suited the show. The cello would be joined later by a violin that may suggests an interplay between different characters.