This very mountainous and dramatic region is interesting for a number of reasons, and not just because it had become the hotbed of Catharism by the 12th century. It is home to many Grail legends, as is the not-far-off city of Marseilles and indeed, Spain itself on the other side of the Pyrenees. It features such towns as Carcassonne and Rennes-le-Château, which notoriously plays a role in such books as Holy Blood, Holy Grail and The Da Vinci Code. And it is from one such castle, the impregnable-looking fortress upon Montségur, where the crusade was brought to a close, but not before (local lore and now wider myth has it), four high-ranking priests escaped down the sheer face of the mountain in the dark of knight carrying some sort of treasure. What could such a treasure be and what became of it?
The whole affair is like to be a fanciful tale, but that has not stopped adventurous historians and treasure hunters from proposing over the years that what escaped Montségur that night was either the Holy Grail itself (whatever it might be) or other invaluable artifacts such as a piece of the "true cross" or the Turin shroud (not yet in Turino, of course!), to some heretical documents or treasures which had been entrusted to the Cathars by the 'Poor Fellow-Soldiers of Christ and of the Temple of Solomon' - i.e., The Knights Templar. The legend goes of course that they had found "something" under the Temple Mount during their early years in Jerusalem, before the ranks of the Order swelled and they were eventually dissolved and persecuted by Pope Clement V and King Philip Le Bon of France (who was of the line of Charles Martel and Charlemagne (the so-called 'Carolingian' kings), not the earlier lineage of the sons of Merovee (or Merovech), who came to be known as the 'Merovingian' kings).
What this "something" the Templars supposedly found during their excavations is unknown to this day, and it's possible the entire story is a romantic fabrication. But there are enough legends and extant, verifiable records from various periods, from the founding charter of the Templars themselves all the way down to the fall of their erstwhile comrades and business partners, the Cathars (and the noble families which propped them up), that it still makes for an exciting read - to me anyway - to read the story of both the Temple Knights and the Cathar heretics, both in its bare historical form and also in various speculative imaginings which excite the mind as to what really happened within the inner ranks of those two seemingly unrelated orders and why the Pope and the Monarchy of France was so eager in both cases to be rid of them.
After all, putting down what it sees as heresy is one thing (though the way in which they did it was so savage and bloody as to remind one of the Roman conquest of that same area more than a millennium earlier) - indeed its treatment of even simple monks who bore no arms would set the tone as well as the doctrinal foundation for the later formal office of the Inquisition - but destroying the Templars, which Papal decree had created in the first place in order to give it a means to fight the Saracens in control of Jerusalem and the rest of the Levant, and which were sworn protectors of the Catholic Church, is less easy to understand.
Well, without creating here an in-depth history of the entire time and place here, I'll just point you in a few directions if this period of history and this series of events interests you.
First, there are a few websites which cover the basics:
And some books (Amazon links given) on the topic:
List of recommended reads
Cathar Castles: Fortresses of the Albigensian Crusade
The Albigensian Crusades
A Most Holy War: The Albigensian Crusade and the Battle for Christendom
The Great Medieval Heretics: Five Centuries of Religious Dissent
Crusade Against the Grail: The Struggle between the Cathars, the Templars, and the Church of Rome
The Cathars: Dualist Heretics in Languedoc in the High Middle Ages
These links were chosen mostly for their relevance to the so-called Cathar heresy.
Feel free to search for books on the Templars, the Holy Grail, or other specialty topics that interest you. There are many!