The Merovingians are descendants of ancient Frankish kings who ruled large areas of what is now known as France and Germany. Clovis I, the head of the house of Merovech, swore allegiance to the Pope in Rome and was baptized, and in return, the Pope named him "the New Constantine." He and his descendants would rule France from 496 to 750, when they were overthrown in a palace coup on orders of the Pope.
But that's just the real-world history you can find in any book on French or European histories. There lies, as with most things, hidden, occult truths to these things.
The origins of the family of Merovech are shrouded in mystery, but it is known that magic ran in their blood. They were occultists, practitioners of magical sciences, and one of many dynasties of sorcerer-kings who ruled the world during the Age of Magic. Where that magic came from is hotly debated. Some believe it came from the fey who inhabited the wild lands of Europe. Others believe the Merovingians were descended from the legendary Minotaur of Crete. Some whispered the Merovingians carried within them the blood of some creature from the deep sea, the strange race modern scholars call the Deep Ones. But, most widely reported, especially by their followers, was that the Merovingians were descended from the Christ himself through his wife Mary Magdeline.
This last claim was a bridge too far. While the Catholic Church was willing to work with the Merovingians to bring peace to their northern frontier, their continued heresy increased tensions between the Pope and the Frankish Kings. Eventually, Pope Zachary sent the Holy Inquisition to end the line of Merovech, and placed Pepin the Short, son of the legendary champion Charles Martel, on the throne of France.
Branches of the Merovingian Dynasty survived the Inquisition's purge and fled into hiding alongside their supporters. They have survived the centuries since, continuing the family traditions of magic and the occult, through the rise of the Veil, the tumultuous Middle Ages, the Protestant Reformation that broke the power of their Papal rivals, the rise of the French Republic and the carnage of the World Wars. While many members of the dynasty are well-to-do nobles, businessmen, scholars and academics scattered across Europe, the house still claims rulership of France, Germany, Switzerland, Austria, and the Low Countries, which they see as unjustly ripped from them by a terrified Pope to scared to admit the truth of their divine blood.
And, as with many secret societies, they have plans to retake that power which they see as rightfully theirs.