The historical and literary study of Jacques Marcal is devoted to the mysteries of the Cathar heresy and the secrets of the castle of Montsegur, their last stronghold in the hopeless confrontation with the king of France and the Roman Catholic Church. What is it about the Cathar doctrine that endows them with such powerful and merciless enemies? What the last initiates hid in the impregnable fortress and managed to endure on the eve of surrender. Was it the Holy Grail, or the treasures of the Jerusalem Temple. And that there is a Grail that the envoys of the Third Reich were looking for here. Did the Templars really patronize the Cathars, for which they paid by inheriting their secret, as well as their fate. And why exactly the castle, whose architecture resembles a solar temple, and located on the slopes of an anciently revered sacred mountain, became a symbol of the secrets and mysteries of the Cathars. Is there really “royal blood” involved here, and its trace will lead us to Rennes-le-Chateau.