Complex sentence: In order to assist the request of their latest client, they hired Barrett to create a weapon, which was what had destroyed the National Art Palace, the weapon is the Silent Noise System AKA the organ Issak was playing and the bells that were ringing.
Split into simpler sentences: In order to assist the request of their latest client, they hired Barrett to create a weapon, which was what had destroyed the National Art Palace. The weapon is the Silent Noise System AKA the organ Issak was playing and the bells that were ringing.