The Priory of Sion
The Priory of Sion is the invisible existence of the entire plot of The Da Vinci Code. The murder of Jacques Sauniere is what triggers Langdon and Neveu to discover the location of the Priory's grand secret for it comes into the hands of their archenemies, Opus Dei.
According to the Priory's "history" in their archives, the Order of Sion was founded in 1090 in the Holy Land by Godfroy de Boullion, who captured Jerusalem in 1099.
The Priory of Sion and the Knights Templar worked together for a time then after a dispute, they broke and went their separate ways. The Knights Templar worked publicly while The Priory went underground, adopting the name the Order of the Rose-Cross Veritas.
In 1965, a list was found contained in the Dossiers Secrets that tells of all of the Grand Masters of the Priory of Sion. Holy Blood, Holy Grail was written in 1982 by Michael Baigent, Richard Leigh and Henry Lincoln. They spent years tracing and dissecting all of the codes and history of the secret society. They never discovered what the real purpose of the Priory was.
High on the list of things to do was the restoration of the Merovingian monarch to France. The coauthors thought the theory that the Merovingian bloodline might be the lineage descending from the children of Jesus and Mary Magdalene. Mary is believed to have found refuge in France after the Crucifixion. Therefore, the main purpose of the Priory was thought to be to protect the holy bloodline of Jesus!
After 1984, the Priory went underground again. On December 27, 2002, an official PoS statement was released, announcing the public re-launch of the society, signed by Pierre Plantard's former private secretary, Gino Sandri, under the title of General Secretary.
And so, the true history and existence of the Priory of Sion is still a mystery today. It makes one wonder if a secret that has been held for almost 1,000 years still isn't known today, is it meant to be known?