This is one of the big mysteries first entioned in the book of Genesis in the Bible, like the Nemphilim, and the Bible is light on the details even though you want to know more about this subject. It's possible to get a bigger picture of what the Order of Melchizedek is about by connecting the dots between various things like astrology, history, mysticism etc,

http://transcendpolarisation.webs.com/theorderofmelchizedek.htm