The Grand Fall is a theory that was established by Galenos, a Craymel Engineer and researcher from Celestia. He discovered that the two worlds of Celestia and Inferia were moving closer to each other, and he postulated that it was due to an increased pull of gravity upon both worlds by the dark matter that was accumulating on the Orbus Barrier. He calculated the collision of both worlds, leading to their mutual destruction. Realizing that such an event could not possibly be caused by normal people, he came to the conclusion that it was somehow initiated by someone with the power of the Dark Aurora, whom he believed to be Balir, the Inferian who had taken over their world. Desiring to stop Balir, Meredy volunteered to travel to the world of Inferia to find a way to defeat Balir and reverse the Grand Fall, with her primary objective of obtaining the Greater Craymels of Inferia to be fringed with the ones that are native to Celestia.
A similar theory was developed by Keele Zeibel, then a student of Light Craymelology at Mintche University. He wrote a thesis detailing his theory of World Chain Destruction (連鎖的世界崩壊仮説?), through which he described the impending destruction of both worlds due to a variety of possibilities he deemed to be beyond his ability to verify without the aid of the Royal Observatory of Astronomy in Inferia City. Among these are the idea that the dark matter is "a side effect of the raised Craymel pressure caused by Grobule distortion", or that "the localized stress from the Craymel Zone has caused a Doctark Effect in the Orbus Barrier". When his thesis encountered criticism from his peers, Keele believed that his arguments were sound, and that everyone else must be basing their criticism on false statements and misunderstood facts.
Unfortunately, Keele's ideas were dismissed and directly opposed by the dean of Keele's college, who reprimanded him for proposing what he viewed as nonsense. Unwilling to give up his theory, Keele was expelled from the university, and he sought refuge in the Mt. Mintche Observatory, using its outdated telescope to conduct his own investigation into the matter, noting that the size of the dark matter was increasing with each passing day, "at the rate of 300 langes a year". Upon meeting Meredy, Keele realizes that her message is as he had predicted, that Celestia and Inferia would collide, though Meredy does not know the specifics that her instructor Galenos had discovered, preventing Keele from verifying the accuracy of his theories.
Regardless, he picks up the term "Grand Fall" from his conversations with Meredy, eventually using it to refer to his own work. During his attempts to spread the information detailed in his thesis in order to start a proper scientific investigation of the matter, he discovers that the religious leaders of the Seyfert Sanctuary in Inferia City believe the dark matter within the Seyfert Ring is a sign of the second coming of Seyfert, seeing it as a blessing and admonishing anyone who would claim that it could be associated with disaster. This leads to the capture of Keele and his allies and the subsequent claim by the King of the Inferia Empire that Keele was spreading sedition in an attempt to cause turmoil in the Inferia Empire, a crime that holds the penalty of death.
Luckily, the death penalty is commuted by Chancellor Claudio Zosimos, who read Keele's thesis after he left it behind at the Royal Observatory. Intrigued by Keele's ideas, Zosimos was able to determine that the two worlds are now 2,000 langes closer than the previous measurement on record, archived from two years before. Zosimos offers Keele a position within the Royal Observatory, allowing Keele to work with him to discover the cause. The pair discusses how it is impossible for the Grand Fall to be caused on purpose since "the Kaloric flow due to Craymel expansion far exceeds the logical limit of the Human Focular Effect", and "according to the Orenash equation, it is impossible to stop the Orbus wave before cellular collapse occurs".
Nevertheless, on the following day, Zosimos declares before the royal court that the Grand Fall must be the result of a malevolent Celestian plot, much like the claim that all of the other disasters that Inferia had experienced in the past were caused by Celestians. Calculating that the collision of the two worlds would occur at around 100 sohme from this time, he stressed the importance of the situation as one of immediate danger. Zosimos said this to boost the matter to top priority among the empire's leadership so that more assets can be contributed toward discovering the true scientific cause and any means of reversal.
After the party travels to Celestia through the Bridge of Light, they make contact with Galenos, the sage who made his own predictions about the Grand Fall. The group learns of Balir's role in initiating the disaster, as the bearer of the Fibrill of Nereid, Inferia's presumed god of destruction, and his relation to Meredy as her father. They decide to confront Balir at his castle, ready to fight him and try to reverse the Grand Fall. However, they discover that Balir has been dead for ten years, and that his consort Shizel is responsible for all of the deeds carried out in Balir's name. They soon discover that they are not capable of withstanding her Dark Aurora, and Reid Hershel decides to undergo the Trials of Seyfert to obtain the Divine Aurora and counter Shizel's powers.
They eventually learn that the intention behind the Grand Fall is to destroy all of Eternia, returning reality to its original state of nothingness that was once Nereid's domain. This is being done due to Shizel's interpretation of Balir's desire for an ideal world beyond the confines of reality, which Shizel came to associate with Nereid's goal of the complete destruction of Seyfert's creations to achieve the restoration of Vatenkeist. Although the party is able to defeat Shizel and Nereid in the end, they realize that the two planets have moved past the critical point, after which the pull of gravity from both planets would continue to force them together and can no longer be stopped.
However, Reid suggests that he could use his Divine Aurora and fringe it with the Dark Aurora, which would have to be controlled by Meredy, enabling them to generate enough force to push the two planets away from each other, effectively destroying Eternia and redesigning reality. Shizel, filled with remorse for what she had done, chooses to take Meredy's place, resulting in her and Nereid becoming sealed within the core of Seyfert Ring as the Orbus Barrier is ruptured, splitting the two planets apart and permanently averting the disaster that they would collide.