For an event as major as the Great Collapse, there is hardly anything we know about this historical catastrophe. We know it marked the end of the Ancient Zemurian Civilisation, which in its entirety disappeared without a trace bar from ruins throughout Liberl and in Grancel’s museum, and plunged the world into five centuries of Dark Ages. A whole continent, split in seven factions, collapsing at approximately the same time. Virtually everything we know about it comes from Sora no Kiseki; both the Crossbell and Erebonia arc hardly contributed to our understanding of the Great Collapse. So, what happened?
In short: the Sept-Terrions gained autonomy.
On the third page of the report “Concerning the Seal Mechanism”, which can be gathered in the Shadow Towers in Sora no Kiseki SC‘s seventh chapter, Celeste D Von Auslese writes that “once it obtained autonomy, the Aureole began to change our society and our lives drastically”. In other words, at one point, inexplicably, the Aureole gained autonomy and affected the ancient civilisation to the extent that Celeste felt the need to draft up a grand-scale plan to abandon the Sept-Terrion her faction’s has relied on for millenniums.
Similarly, in Ao no Kiseki‘s Fragments chapter Zeit shares his knowledge about his duty to oversee former Crossbell civilisation’s Sept-Terrion, the Demiourgos. As presumably millenniums passed, the Demiourgos realised its own spirit started to decay and decided to end its own existence before it would end up hurting the people it was supposed to protect. This, too, happened around the same time the Liberlians decided to seal the Aureole (after 700 years, alchemy was invented and it took the alchemists another 500 years until the Demiourgos was re-created). Like the Aureole, neither meant malicious intent, yet both gained autonomy to the extent that they were able to think, judge and decide.
Whereas the two Sept-Terrion in Erebonia have yet to be revealed,it is commonly assumed that they are (linked to) the two fighting Giants mentioned in Sen no Kiseki II‘s Black History. Let’s re-read the volume assuming the Giants are indeed the Sept-Terrion. Two factions descended from the heavens with their Sept-Terrions, for whatever reason, and met one another, unexpectedly. The two Sept-Terrion engaged into a fight that even scared their followers, suggesting the Sept-Terrion behaved differently than they were used to. In the end, they exploited and exhausted each other until they ran out of energy. The goddess, who had given the Sept-Terrion to the people millions of year before, laments their fight. It were no longer the peaceful Sept-Terrion she had once given to the peoples. The final sentence describes an “enormous strength” left behind, suggesting the remnants of the Sept-Terrion in Erebonia.
Here, we see three cases in which the behaviour of the Sept-Terrion suddenly changed: the Aureole explicitly gained autonomy, the Demiourgos decided to take its own life and the two Sept-Terrion of Erebonia suddenly engaged into a fight as they descended on earth. So while the descriptive Great Collapse could refer to the collapse of human’s control over the Sept-Terrion, we have still no inkling as to what caused these sudden sparks of autonomy among the Sept-Terrion that led to the Great Collapse.