The pilot reported that he was conducting helicopter external sling load operations to a remote mountain top. At the drop off site, while in a hover about 20 ft above the ground, with his attention focused outside to ensure he cleared terrain and obstacles, he felt the helicopter begin to settle. The rpm began to decay, the low rotor rpm light illuminated, and the horn sounded. He immediately lowered the collective, released the cargo and impacted the ground coming to rest on top of the cargo. The helicopter sustained substantial damage to the rotor system and vertical stabilizer. The pilot reported that there were no preaccident mechanical malfunctions or anomalies with the helicopter that would have precluded normal operation, and that the accident may have been prevented if he had received additional training from the company to include flying at gross weight with and without external loads. In addition to having pilots check for available power in an out-of-ground effect hover with altitudes and escape routes before committing to the approach.