The 6 Grandfathers are based on the religious beliefs of The Oglala Sioux American Indians also later known as The Lakota Sioux. 'Black Elk Speaks' a Novel written by John Neihardt in 1932, shares the life narrative of Nicholas Black Elk, a Holy Man of the Oglala Sioux. Accounts are given on Black Elk's great vision of things to come for the Lakota Sioux. For more on the Lakota Sioux's Religion and Culture click on the link. Also a Great movie to watch although only fictionally related would be "Little Big Man"