What is the Priory of Sion? What's the Connection to the Holy Grail?

What is the Priory of Sion?:
Also known as the Prieuré de Sion or the Priory of Zion, the Priory of Sion is commonly used in works of fiction as a powerful, secretive organization that has been behind important events throughout Western history. Even some works purporting to be factual have included this claim and Dan Browns The Da Vinci Code blends both: a work of fiction claiming to be based on historical fact, incorporating the claim that the Priory of Sion has been behind such things as the Knights Templar.

Dan Brown on the Priory of Sion:
According to Dan Brown, “the Priory of Sion — a European secret society founded in 1099 — is a real organization. In 1975 Paris’s Bibliothèque Nationale discovered parchments known as Les Dossiers Secrets, identifying numerous members of the Priory of Sion, including Sir Isaac Newton, Sandro Botticelli, Victor Hugo and Leonardo da Vinci.” This is one of the examples of Dan Brown claiming his fiction is based upon fact, but what he claims to be fact is itself a hoax.

The Priory of Sion, Jesus, and Mary Magdalene:
What is the Priory of Sion responsible for that they play such an important role in Dan Brown’s fiction? According to Brown (and a few others), the Priory of Sion has kept the secret that Jesus and Mary Magdalene married, had children, and that their descendants went on to become the Merovingian dynasty based in southern France. This “sacred” bloodline survives down to this day and it’s a secret which the Catholic Church would kill to keep hidden.

The Real Priory of Sion:
There was a medieval monastic order known as the Priory of Sion, but it died out and all its assets were absorbed by the Jesuits (Society of Jesus) in 1617.

The Modern Priory of Sion:
It’s common for modern movements to claim an ancient pedigree, so when the anti-Semitic Vichy sympathizer Pierre Plantard created a new organization in 1956, he called it the Priory of Sion and claimed to be the last heir to the Merovingian dynasty. Plantard claimed that Les Dossiers Secrets linked the modern group with the ancient one. Under oath, he admitted that the documents were forgeries, but this has not stopped authors like Dan Brown from recycling the claims.