The commercial helicopter pilot reported that the purpose of the flight was to transport two technicians to a remote site in an area of mountainous terrain. The pilot fueled the helicopter and flew approximately 30 minutes to a staging area. After loading the two passengers and their gear, he flew to the planned landing zone (LZ), in a narrow box-valley at about 2,550 feet MSL. The proposed LZ was evaluated while performing an out-of-ground-effect hover, but the pilot determined the LZ was unsuitable. A second LZ was evaluated using the same process and also found unsuitable. While hovering out of ground effect, above a third potential LZ, the helicopter began to descend and the pilot added collective pitch to stop the descent. He said the low rotor annunciator horn then sounded and the helicopter continued to descend. In an attempt to avoid landing in an area of uneven terrain, the pilot said he opted to turn the helicopter to the right and downslope, while trying to override the engine governor to attain additional engine power. The helicopter's skids subsequently struck an area of uneven, down sloping terrain. During touchdown, the helicopter nosed down, pivoted to the left, and rolled onto its left side. The helicopter sustained substantial damage to the fuselage, tail boom and main rotor drive system. The pilot reported no preaccident mechanical anomalies with the helicopter that would have precluded normal operation. The pilot characterized the down sloping wind conditions as light and variable.