The Priory of Zion or Prieuré de Sion is, supposedly, a secret society based in France.
Baigent, Leigh, and Lincoln claim that it has a long an illustrious history, that it is dedicated to returning the Merovingian dynasty that ruled the Frankish kingdom before A.D. 751 to the thrones of Europe, and that the order protects these kings because they are the literal descendants of Jesus Christ.
Baigent, Leigh, and Lincoln also claim that the Prieuré had a large role in partaking in and promoting the "underground river of esotericism," the Alph, in Medieval Europe.
Many historians who specialize in the area do not accept their work as a serious contribution to scholarship.
--- Sources ---
Michael Baigent, Richard Leigh, and Henry Lincoln. The Holy Blood and the Holy Grail, 1982.
see also conspiracy theory