The Kharlan War, also known as the Ancient War, was a conflict that began 5,000 years prior to the events of Tales of Symphonia and lasted for 1,000 years. The war was a confrontation between two powerful countries, Sylvarant and Tethe'alla. It was ended by Mithos the Hero in the Holy Ground of Kharlan.
During the events of Tales of Symphonia, it is revealed that Mithos Yggdrasill, a half-elf, along with his sister, Martel Yggdrasill, fellow half-elf, Yuan Ka-Fai, and human, Kratos Aurion, managed to stop the war by forging pacts with the Summon Spirits. After the war ended, the group was betrayed by an unnamed human, and Martel was somehow killed in the process. Mithos became unstable, and he told Origin, ruler of the Summon Spirits, to make him a sword capable of manipulating creation and time itself. Origin trusted Mithos and created the Eternal Sword, which Mithos used to separate the world in two, thus becoming Sylvarant and Tethe'alla, named after the two former countries.
However, Mithos broke his vow with Origin and sealed Origin within Kratos. By doing so, the pact between Mithos and Origin would only be broken if Kratos were to die. Mithos then created Cruxis, an organization that would govern the worlds like gods, on Derris-Kharlan to find a way to revive Martel, whose soul was affixed to the Great Seed through a Cruxis Crystal. Mithos proceeded to make himself, Kratos, Yuan, and figuratively Martel the leaders of Cruxis, known as the Four Seraphim. They used the power of Cruxis Crystals to grant them possible immortality, and for over 800 years, Cruxis used the Journey of Regeneration to assure that neither world would exhaust the mana left by the Great Seed. The Chosen One soon became a ritualistic pawn in the scheme, as their mana signatures are closely related to Martel's. Chosens are the result of marriages arranged to produce host bodies for Martel's soul.