As the pilot was descending from 7000 to 5000 ft, he heard a loud bang, followed by a temporary vibration and a violent swerve to the right. He looked out the right window and thought the right engine had fallen off since all he could see was the tops of the engine mounts. The aircraft was controllable only after reducing the left engine power to idle. During a wheels up/forced landing, the plane tore through a fence, slid across a paved road, went over an embankment and began burning after it came to rest. The pilot received a back injury and had difficulty egressing. An investigation revealed the propeller hub had failed from fatigue which resulted in blade separation and subsequent engine tearaway. The engine had remained attached to the airframe by cables. Metallurgical examination of the hub revealed that the fatigue originated at a threaded grease fitting. Gouging was visible on the inner wall. A set of incorrect counterweights were found on the propeller. The counterweight's function was to reduce vibration.