They have been maintaining the balance of Yin and Yang since ancient times. Whenever the world is in chaos, there are many cases where evil spirits come to do harm to humans. The gods select talented humans and they are given their own guardian god for them to calm the over flow of Yin. This has continued all the way up to the present, so those that are chosen by god are hunters that eradicate ghosts, they are called Ghost Hunters. (Source: MU)