The twenty-year genocide of the heretical Cathars throughout the Languedoc region of Southern France was followed by an additional 130 years of inquisitional weeding out of what remained of these heretics, which meant that their elimination was inevitable.
With the exception of a few remote communities scattered throughout the Pyrenees, the last significant stronghold of the Cathars in France was Montségur. It's capture would mean that Catharism in the South of France would fundamentally cease to exist.
Besieged for ten months, the Cathars resisted repeated assaults, but by March 1244 they surrendered. It is said that just over 200 Cathar perfecti (male and female priests of the heretical movement) chose martyrdom over a conversion to Catholicism. According to popular local myth, they were corralled into a wood-filled stockade at the base of the mountain, and without resistance were incinerated.
Today, at the base of the mountain is a modern stele located in an area called Prat dels Cramats (Occitan for 'Field of the Burned'). On it is inscibed in Occitan, Als catars, als martirs del pur amor crestian. 16 de març 1244 ('The Cathars, martyrs of pure Christian love. 16 March 1244').
Rumours abound that the night before the incineration, four perfecti managed to escape into a nearby forest to retrieve Cathar treasures hidden there. Legends linking the Cathars and the Holy Grail have lead to the notion that the retrieved Cathar treasure of Montségur may have actually been the Holy Grail.
Given that their core religious belief ultimately saw wealth as an abomination, it can be concluded that their treasure had nothing to do with material wealth. Medieval paintings that included the Cathar frequently showed them carrying books, which implied that knowledge was of primary importance.
It would be prudent to assume, that their treasure may have actually been manuscripts, and what was on them must have been of highest importance to the perfecti. Given the core docterine of the Cathars theology was dualism, the most sacred information to protect would be the teachings on how to escape 'the world of the evil god', so that one may enter 'the world of the good god', resulting in enlightenment, or as they called it 'returning to the kingdom of heaven.'
History also tells us that the Cathar had a particular interest in the Sermon on the Mount, which included the Beatitudes. The essence of the Beatitudes describes a process of emptying yourself of attachment to worldly values such as wealth, power, special love, and success, and replacing them with spiritual values such as charity, being a peacemaker, and perpetual forgiveness.
Manuscripts and teachings such as this would surely end the Catholic Church as their desire for wealth and power had truely taken precedence over their desire to embody spiritual values, hence the crusade and persecution that took the lives of hundreds of thousands of Christians.