The history of the known world in the Kyoukai Senjou no Horizon series picks up from the events in Owari no Chronicle, with the world destroyed due to the effects of powers that were possessed by the people at that time. At this period, the inhabitants of earth turn to a mysterious book and its record of the distant past to try and retrace their steps in an attempt to prevent the mistakes they committed.
In an attempt to save the Earth and its alternate versions from collapse, a group of people sought to migrate people and their concept cores — objects which dictate the rules of a certain world — onto the only safe Earth available. Although the process of transfer was successful, it also caused Earth to become a dangerous place to its inhabitants due to the multiple concepts creating conflicting laws and thus, making inhospitable conditions for humans. Because of this, the inhabitants of Earth began a journey towards a realm they call "the heavens" in order to look for a new home.
Throughout the journey of Earth's inhabitants, a war erupts amongst all the existing realms, and only the current world was left standing. This tumultuous event forces humans to return to Earth, which has turned into an inhabitable planet except for a certain area called the Divine States. On this land they remained, unable to cross the seas or venture beyond the skies due to the hostile environment that await them.
As the population of people rose, the need for land increases, and it becomes a trigger for another armed conflict. This time it involves the inhabitants of the Divine States and those from other countries. To solve the problem of land, the Divine States were duplicated in an area called the Differing Space, and the Harmonic Realm was born. Most of the humans crossed over to the Differing Space, and they try to experiment on the new land they created, while examining the Divine States closely. Meanwhile, a set of powerful items called Divine Tools were entrusted to the inhabitants of the Divine States.
During the progress of time, doubt slowly developed as to whether ascending to "the heavens" was even possible. As a result, a history book called Testament was created. It serves as a guide for the people to retrace the steps of the former mankind throughout history.
History Recreation PeriodEdit
Using the Testament as basis, the inhabitants of the real world begin the retracing of history from the year 10,000 BC, the Old Testament. They performed the history recreation of the years 10,000 BC to 1000 BC within the span of only 90 years, while the following 1,000 years were reenacted within a century. Starting from 1 AD, this meant the beginning of the New Testament and the Testament Era (TE) with the history recreation process is slowed down to the ratio of 1 AD is to 1 TE.
History reproduction proceeded smoothly until 1413 TE, when the Nanboku-cho War was recreated in the Divine States. During this war, the Southern Court initiated a rebellion. They stormed the Imperial Castle and usurped the Divine Tools from the Emperor. Those three treasures controls the world's ether vein. As a result, the Harmonic Divine States in the Differing Space, which had its stability supported by the ether vein, collapsed. More than half of Harmonic Divine States was destroyed. The remnants of it became fused with the Divine States and overwrote most of it, creating Harmonic Territories.
In response to the destruction of their home, the people from the Harmonic Divine States invaded the Divine States, intiating the Harmonic Unification War. The people of the Divine States, whose resources were depleted after the recreation of the Nanboku-cho War, surrendered.
According to the Testament, the Divine States is not supposed to be taken over by another territory; also, the Divine States refused to let their military and government be controlled by outsiders. As a way to solve this dilemma, the Divine States was split into the different Harmonic Divine States and the remaining land of the Divine States is then called the Far East. In addition to this, each country's military and government was replaced by an academy, an educational institution. The leadership of each country would be split between representatives of a Harmonic Divine State and the feudal lord in that region. Also, the wars that are recreated from history would now become battles between academies, turning history recreating itself as a negotiation card.
With this laid out, the process of history recreation continued from 1457 TE. However, the update of history terminates in 1648 TE. A rumor of an imminent apocalypse began to spread around the world.
In what appears to be the year of the apocalypse, a boy named Toori Aoi faces the grim reality of the death of his friend Horizon Ariadust. Along with his declaration of his feelings to a mysterious automaton who looks like Horizon that turns into a declaration of war to the many territories that currently repress his homeland, he and his friends began a quest to conquer the world and save it from the impending Apocalypse.