Mysterious Masonic-style secret society allegedly founded by crusader Godfroi de Bouillon in 1099 in Jerusalem. It is said that the Knights Templar were part of them for a long time, being the military arm of the Prieure. A serious dispute occurred around the time the Templars failed to prevent the Saracens from reconquering Jerusalem in 1188, and the orders were sundered, and the Templars continued on their way until they were finally suppressed by King Philip IV of France and Pope Clement V.
In the meantime, the Prieure has continued on its mysterious business, some of its goals allegedly being to establish a Merovingian descendant on the throne of France, and to establish a European Union. There's probably something more than just that. Whatever their true goals are, they seem to have attracted many very interesting characters in their long history: Nicolas Flamel, Robert Fludd, Sandro Filipepi (better known as Botticelli), Leonardo da Vinci, Robert Boyle, Isaac Newton, Victor Hugo, Claude Debussy, and Jean Cocteau are all listed as being Grand Masters of the Prieure at one time or another, and looking closely at their backgrounds does not show this to be as unlikely one might initially think (for instance, contrary to everyone's image of Newton as a no-nonsense scientist, he really was steeped in a lot of occult studies).
The theory about them goes something like this. Jesus Christ really had been a recognized King of the Jews who had attempted to overthrow the Romans. He was supposedly married to Mary Magdalene, who eventually migrated to the Languedoc in southern France after the crucifixion, and settled there. Jesus' descendants through the Magdalene eventually intermarried with the local population, and the Merovingian kings were their most important descendants. Eventually, around the 5th Century, the Roman Church made a pact with the Merovingian king Clovis that was supposed to grant them perpetual kingship over the Franks with their blessing. But in the 7th century, the Church betrayed that pact when they endorsed the assassination of Dagobert II by the forerunners of the Carolingian Dynasty. But Dagobert II left a son, Sigisbert IV, whose line continued among the princes of the Languedoc.
Eventually, that Merovingian line produced Godfroi de Bouillon, the great crusader who conquered Jerusalem during the First Crusade. They thought to create a Christian kingdom in the Holy Land that would be the equal of all the reigning dynasties of Europe. Unfortunately, due to the failure of the Templars to maintain control of Palestine, all those hopes were destroyed, and they were forced to change their plans.
The Prieure de Sion then began to pursue its continuing agenda to install a Merovingian descendant on the throne of France throughout the next thousand years, by operating in secret. Among these families that were their sponsors were the houses of Guise and Lorraine, and later the Habsburgs (who had married into the house of Lorraine), who were the beneficiaries of two failed attempts to overthrow the reigning French monarchy. The Fronde insurrection was one such attempt, going head to head against Louis XIV and Cardinal Mazarin. The painter Nicolas Poussin seems to have been involved in all this, as he had incorporated the society's motto: Et In Arcadia Ego in several of his paintings, most notably Les Bergers d'Arcadie, which Louis XIV had taken and sequestered in his own apartments where no one could see it, for some reason. In the 19th century they had worked hard to make the Hapsburgs emperors of all Europe.
In the late 19th century they were at the center of a mystery in a small village in the Languedoc known as Rennes Le Chateau and its parish priest Berenger Sauniere. Sauniere had apparently found a cache of important documents in the crypt beneath the church when he planned to renovate it. After sending the documents to a priest in Paris named Emile Hoffet, and getting acquainted with his circle of friends which included the singer Emma Calvé and (the alleged Prieure Grand Master) Claude Debussy, he became inexplicably wealthy and embarked on some very strange building projects, including a very unorthodox renovation of the old church and building of a large tower dedicated to the Magdalene. It was from investigating this mystery that Michael Baigent, Henry Lincoln, and Richard Leigh wrote their famous book, Holy Blood, Holy Grail, from which much of this writeup is based.
It's a bizarre story, I know...