Complex sentence: There is a magnificent concert organ set as a background to the stage, which was designed to complement the surrounding architecture of the Hall and installed in 1991 in time for the 10th anniversary of the NCH's opening.
Split into simpler sentences: There is a magnificent concert organ set as a background to the stage, which was designed to complement the surrounding architecture of the Hall. It was installed in 1991 in time for the 10th anniversary of the NCH's opening.