Schism Wars

The Schism Wars, also called the Wars of Schism or simply the Great Schism as well as many other names, were a series of catastrophic religious conflicts among the humans of Orithea roughly five thousand years before the present day. They were driven by the clash between the established cults of the Thaedonu-Vaati (up until that point, the universal and unquestioned religion of all civilized humans) and the new teachings of the prophet Tarquenus, who led his followers to turn away from Utara-Vaati and instead worship the gods of the Rhantaseppa. 
The resulting wars devastated the regions all around the Sarpic Seas, which at that time encompassed most of human civilization, and cost countless lives. Huge armies and fleets were marshaled, massive battles were fought and dreadful slaughters perpetrated. At the climax of the wars, the high priests of both factions called down titanic destructive powers on each other, and legend tells that the gods themselves descended from Aenuvion to fight on the side of their worshippers. Cities were obliterated, fire rained from the sky, the earth shivered, lands sank beneath the churning seas. 
In the end, Tarquenus and his followers realized they were beaten and faced nothing but annihilation if they stayed in Orithea. The prophet ordered the Rhantashani to construct a fleet of vessels and they took to the ocean, sailing east towards unknown lands. They had become refugees who would be known as the People of The Ten Thousand Ships. After long and difficult journeys, the survivors made landfall on the west coast of Achuirea, where they settled and began exploring and expanding deeper into this strange new continent. The modern-day Achuireans are, for the most part, the distant descendants of these Tarquenian exiles, and they persist in the worship of the Rhantaseppa to this day. 
The era before the Schism Wars is considered the archaic period of human history, about which contemporary scholars know relatively little. This is partially a function of its great remoteness in time, and partially a result of the widespread destruction of records which occurred during the wars; many great temples and libraries, with all the books and manuscripts they held, were lost as cities burned, and especially during the devastating upheavals of the wars’ final phase.