|Then, once the Eldian Empire ran out of enemies, they began to kill one another. That was the beginning of the Great Titan War. The families who held the eight Titans shed each other's blood in an endless cycle of conspiracy and betrayal.|
|— Willy Tybur recounts the start of the Great Titan War|
The Great Titan War (巨人大戦 Kyojin Taisen?) was a period of civil war within the Eldian Empire. The 145th king, Karl Fritz, abandoned Eldia's conflicts, which led to fighting between the other eight houses with the power of the Titans. This allowed the oppressed Marleyan people to rise up, and, with support from the Tybur family, they overthrew the weakened Eldia. The war resulted in the fall of the Eldian Empire, the retreat of the Eldians to Paradis Island, the creation of the Walls, and the restoration of Marley.
For many hundreds of years, the various familial houses subservient to the Fritz family were in constant conflict between each other for control over the eight Titans, though the power of the Founding Titan maintained order in the Eldian Empire throughout the centuries. However, when the Founding Titan was inherited by Karl Fritz, the 145th King of the Fritz monarchy, he chose instead to abandon the conflict of Eldia and relocate the capital to Paradis Island to the east of the mainland. Believing that Eldia deserved retribution for their crimes against the world throughout the centuries, Karl Fritz conspired with the Tybur family, the holders of the War Hammer Titan, to bring about the downfall of Eldia. In the King's absence, the people of Marley, descendants of an ancient nation defeated by Eldia, seized the opportunity to take control of the war, inspired by the tales of the Marleyan hero Helos, secretly a fabrication created by Karl Fritz and the Tyburs.
After the Eldian Empire began to run out of enemies, the eight families who held the Titans turned on each other in an endless cycle of conspiracy and betrayal. Amidst the conflict, the nation of Marley succeeded in gaining the powers of seven out of the Nine Titans which held dominance in Eldia, dramatically turning the tide of the war in their favor. The first of these Titans to support the Marleyan uprising was the War Hammer Titan, held by the Eldian Tybur family. It is unknown for how long the Great Titan War was waged, but during the conflict Marley succeeded in claiming nearly all the territory that had previously belonged to the Eldian Empire.
According to Willy Tybur, King Karl Fritz grew tired of the constant conflict between the houses, and pitied Marley for its history under the empire's tyranny. By conspiring with the Tybur family, King Fritz fabricated the hero Helos to present Marley as the nation that led to the Eldian Empire's downfall. Unaware of this conspiracy, the Eldians continued to eliminate each other for some time.
In the year 743, Karl chose to make use of the Founding Titan to put an end to the war and bring about lasting peace for Eldia. Eventually, after gathering the people of Eldia, including non–Subjects of Ymir, to Paradis Island, King Fritz used the Founding Titan to guide countless Colossus Titans in the construction of three concentric Walls around Eldia's remaining territory: Maria, Rose, and Sheena. In order to cease Marley's advance, King Fritz sent an official ultimatum stating that if they dared to make any hostile move against the people of the Walls, the Colossus Titans within would be unleashed upon the world to "flatten the entire earth." When all of Eldia had gathered within the Walls, the gates were sealed and King Fritz sent a message to the Fritz family who remained in Marley, stating that he had made a vow with the Founding Titan renouncing all war. With this vow, the Great Titan War came to a close.
After the closing of the gates in the Walls, Karl Fritz used the power of the Founding Titan to erase the memories of the world before the raising of the Walls from the minds of the Subjects of Ymir. For the next century, the Subjects of Ymir would live on believing themselves to be the last remnants of humanity on earth, with the world beyond the Walls overrun by the Titans that had appeared without warning in the year the Walls were raised.
The nation of Hizuru was called into question because of their alliance with Eldia and had their reputation tarnished as a result of the war with a branch of their Shogun clan being left behind on the island.
Across the sea on the mainland of the newly reformed nation of Marley, the people of Eldia who were left behind on the continent became low-class citizens under the authority of their new rulers. Rather than exterminating the Eldians, the government of Marley chose instead to confine them within the internment zones, such as in Liberio, separate from the Marleyan populace. The Tybur family was the only Eldian house that was allowed to keep their Titan because they went against the Eldians and allied with Marley. They were also granted positions of high nobility as honorary Marleyans and were permitted to live on luxurious estates. Some remnants of the Fritz family also remained in Marley, displeased with King Fritz's decision to abandon the conflict of Eldia. For the next century to come, the people of Marley would believe they had truly conquered Eldia by their strength, unaware of Karl Fritz's actions in bringing about the downfall of his own nation. Over the next eighty years, some Eldians would come together to form a resistance movement with the goal of overthrowing Marley and restoring the empire of Eldia.
In the years after the Great Titan War, the Marley government would slander the old Eldian Empire with the claim that they had resorted to eugenics and ethnic cleansing during their reign, when such a catastrophe might not have ever happened. However, Karl Fritz does acknowledge the nation of Marley was harshly persecuted under Eldian rule.
One hundred and seven years after the end of the Great Titan War, the Survey Corps discover the basement of Grisha Yeager, an Eldian of the Marley internment zones who came to live within the Walls; in Grisha's journals they learn the true history of the age before the raising of the Walls, as well as Grisha's past before his arrival at Paradis Island.
Four years later, in the year 854, Willy Tybur chooses to reveal the truth of the Great Titan War to the world in a stage production held in the internment zone of Liberio. In this production, the Tybur family reveals the fabrication of the legend of Helos and the true role Karl Fritz played in the destruction of the Eldian Empire.