Inferia and Celestia are connected by a divider known as the Orbus Barrier, which yields the Seyfert Ring.
In Tales of Eternia: The Animation, it is revealed that centuries ago, the two worlds had come into contact at some point. However, the differences in faith and built-in contradictions led to a deadly conflict known as the Aurora War (極光戦争?). Miraculously, war was eventually put to a halt. As punishment for invading their home, the Inferians banished the surviving Celestian soldiers who were left behind, to the archipelago of Belcagne. Unable to handle the shame of defeat, the Celestians on Belcagne would swear revenge. To this end, they built a destructive cannon called the Wedge of Punishment. As a means to keep their hatred for Inferia alive, the adults brainwashed one of their children, Excia, into believing that destroying Inferia would save their home world of Celestia and used the remaining Celestian technology they had to split her consciousness in two, making her immortal.
Ever since the end of the Aurora War, contact between the two worlds has been absent for centuries, until Meredy manages to slip through the barrier, initiating the events of the game's plot. The protagonists manage to cross over from Inferia to Celestia via the Bridge of Light, a one-way teleport in Inferia that was discovered around thirty years prior. Later in the story, the group uses the Van Eltia to travel in between the worlds, which are polar opposite planets yielding different elements that result in vastly different atmospheres.
A theory known as the Grand Fall was conceived by the Celestian researcher Galenos, who discovered that the two worlds were gradually moving closer to one another due to an increased pull of gravity generated by the dark matter accumulating on the Orbus Barrier. One of the protagonists, Keele Zeibel, writes a thesis on the topic, naming it World Chain Destruction, in which he speculates the same, though few believe him. Once the group learns more about the Grand Fall and its inevitable nature, later confirmed by Chancellor Claudio Zosimos, they make it their goal to prevent. Eventually, the protagonists make contact with Galenos and learn of Balir and his role in initiating the disaster. They decide to confront him in his castle, only to discover that he has been dead for ten years, as well as that his consort Shizel has assumed his place in perpetuating the Grand Fall, which is revealed to be a plot to end Eternia, returning all of existence to nothingness.
Shizel acts upon her interpretation of Balir's desire for an ideal world, which results in her plans being tethered to that of Nereid's. Nereid is the god of destruction who once ruled the spirit world of Vatenkeist, which the god of creation, Seyfert, sealed away, by creating the physical planets of Inferia and Celestia, manifestations of life itself. The Grand Fall would allow for Vatenkeist to be restored through the destruction of Inferia and Celestia, but in their attempt to prevent this and stop Shizel, the group realizes they are powerless against her Dark Aurora. They undergo the Trials of Seyfert, and Reid Hershel obtains the power of the Divine Aurora, which can be used to counter Shizel's Dark Aurora. The group succeeds in defeating Shizel only to discover that the planets have moved past the critical point, and that collision is now inevitable once the pull of gravity draws the planets together.
Ultimately, Reid fringes his Divine Auroria with that of the Dark Aurora, which Shizel, now remorseful for her actions, decides to control in cooperation. This results in generating enough force to push the two planets away from each other, effectively destroying the existence of Eternia and redesigning reality. Shizel and Nereid are sealed within the core of the Seyfert Ring before the Orbus Barrier ruptures, the final step in ensuring that the planets of Inferia and Celestia remain split apart, permanently averting the disaster of collision.