The Priory of Sion is allegedly a secret organization which holds the truth behind the origins of Christianity and the secrets of Christ, made famous by the books "The Holy Blood and the Holy Grail" by Baigent, Leigh, and Lincoln and "The da Vinci Code" by Dan Brown. It was suppousedly founded in 1099 by the French king Godfrey of Boullion, who was a descendent of Christ and Mary Magdelene, as was the entire royal Merovingian bloodline. The Knights Templar were associated with them becuase they held the secret documents about Christ and Mary Magdelene. Some of its most famous members were Isaac Newton, Victor Hugo, Botticelli, and Leonardo da Vinci, who apparenly presided over the Priory of Sion from 1510 to 1519.
But is this story true? Due to the popularity of the books noted above, it is believed to be, but is not. The true origin of the Priory of Sion is that it was originally founded by a man named Pierre Plantard in 1956, and then began to act in the 1960s. He believed that a hidden royal bloodline existed in France, and he wanted to restore its rightful power. So, he and his associates attempted to achieve their goals by planting false documents in libraries, such as the National Library in Paris. These documents told the false story of the Priory of Sion and that is where the idea of the Priory of Sion began. The documents were proven fake by testing the paper which showed it was from the 1960s.
Source: "The Truth Behind the da Vinci Code" Richard Abanes