The KKK, The Priory of Sion, The Order Of Assassins - the names might be familiar but - Who were they? What were they? What were their beliefs and aims? This six episode series explores some of the most secretive and dangerous global underworld societies from history. It's a murky, mysterious mixture of sectarian cults, fanatical religious groups, esoteric societies and political militants - all of them seeking to shape and influence the world we know today.