Priory of Sion

The main point of the most popular version of the Priory of Sion conspiracy theory dating from the 1980s (and having no relation to the original group founded in 1956) is that Jesus Christ married Mary Magdalene, and that they had children and moved to France. Their descendants became French kings, while the Catholic Church sought to suppress the information. Various bodies, groups and persons throughout history were possessors of this secret knowledge - which was never leaked to outsiders, despite the amount of paperwork that would be needed to keep track of who the heirs were and the likelihood of people gossiping or letting slip information on the subject. These include the Cathars, Leonardo da Vinci (when not inventing flying machines), Father Bérenger Saunière (of Rennes-le-Château, to whom the Vatican was allegedly prepared to pay blackmail monies, rather than have sent to the local institution for the heretically delusional), and the Priory of Sion.

Revelation
The truth of the tale was, seemingly, revealed in Holy Blood, Holy Grail/The Holy Blood and the Holy Grail. Dan Brown's The Da Vinci Code also tells the story in a fictional form.

History
The Priory of Sion was founded in 1956 by Pierre Plantard, in order to claim that Plantard was of Merovingian descent, as well as a grand monarch from one of the prophecies of Nostradamus. Unfortunately, some people took the man's claims seriously, and started believing the made-up history Plantard spouted.

Rennes-le-Château
Rennes-le-Château is a castle village in the Aude département in Languedoc, France, and is the centre of various conspiracy theories.

It is claimed that Father Bérenger Saunière, a 19th century priest, who had undertaken an extensive restoration project on the local church, had financed his activities through blackmailing the Vatican after finding information on the bloodline of Christ. Links were also made to the Knights Templar, Priory of Sion and various other subjects of conspiracy and creative history.

Marketing pitch
It appears that the story was a marketing pitch by a local hotelier, Noël Corbu, in the 1950s, further developed by Pierre Plantard and subsequently others, and the actual source of Saunière's money was the selling of masses (at an inappropriate level).