One of the most unique experiences in the world that can only be found in Mongolia. Mongolian shamanism is an all-encompassing system of belief that includes medicine, religion, a cult of nature, and a cult of ancestor worship. In 13th century, the leading shaman declared Genghis Khan the representative of Mongke Koko Tengri (the "Eternal Blue Sky"). With this declaration of divine status, it was accepted that his destiny was to rule the world. Religious tolerance was practiced in the Mongol Empire; however, to defy the Great Khan was equivalent of defying the will of God. Central to the system were the activities of male and female intercessors between the human world and the spirit world, Shamans (böö) and Shamanesses (udgan). Yet, they were not the only ones who could communicate with the spirit world. Nobles and clan leaders also performed spiritual functions and so did commoners. The hierarchy of Mongolian clan-based society was reflected in the manner of worship.