Shamanism was the ancient religion of the Mongols during the Mongol Empire of Genghis Khan. Shamans acted as intermediaries between humans and gods like Tengri, the god of the blue sky, by entering a trance state to communicate and cure sickness or exorcise spirits. While shamanism included worshipping nature spirits and guardian spirits, it declined as the Mongol Empire weakened and was eventually replaced by Tibetan Buddhism as the dominant religion in Mongolia.