Question: "What was the Merovingian dynasty?"
Answer: Historically, the Merovingian dynasty was a succession of Frankish kings who ruled over parts of modern day France and Germany from the 5th to 8th centuries A.D. The Merovingian kings perpetuated myths concerning their being of "divine descent," as demonstrated by their continued military successes. The authors of Holy Blood, Holy Grail and The Da Vinci Code run with this myth, and concoct a connection between Jesus' supposed marriage to Mary Magdalene and their supposed descendants. The hypothesis is that Jesus' descendants intermarried with the Merovingians, giving the line of kings a divine authority.
This is a baseless theory. There is no record of any of the Merovingian kings claiming their heritage to Jesus. In fact, the majority of Merovingian kings were Roman Catholic - the exact opposite of what one would expect from the theories of Holy Blood, Holy Grail and The Da Vinci Code. According to the conspiracy theories in those books, the Roman Catholic Church covered up the truth about Jesus and His bloodline. This is very strange considering the historical fact that the supposed descendants of Jesus were in fact practicing Roman Catholics themselves. Just as they warp the true identity of Opus Dei and the Priory of Sion, Holy Blood, Holy Grail and The Da Vinci Code take history, in this case the Merovingian Dynasty, and twist it beyond recognition in order to support a wild and baseless conspiracy theory.