Years are numbered in a single calendar, written here as "Year 845" and so on. The present of the game is Year 850, after the 57th Exterior Scouting Mission and before the Assault on Stohess. The three main protagonists were all born in Year 835, so they are ten years old in Year 845 and fifteen years old in Year 850. A cadet enlists at about age twelve and graduates after three years of training, which places the 104th Cadet Corps' enlistment in Year 847 and its graduation in Year 850.
This section is the true history of the world. Almost no one inside the Walls knows any of it. It exists so the DM understands why events unfold as they do.
Roughly 2,000 years before Year 845, a slave girl named Ymir Fritz, owned by a king of the people who would become the Eldians, was hunted in a forest as punishment for a minor act of defiance. Wounded and fleeing, she discovered a giant tree with a hollow at its base and an organism inside it. By contact with that organism she gained the power of the Titans and became the first Titan. She used the power in service of her king, working vast changes on the land: clearing and farming barren ground, building roads across the continent, and bridging mountains. She bore the king three daughters, named Maria, Rose, and Sina. After thirteen years she died, placing herself between the king and a spear and choosing not to use her power to heal. At the king's command her three daughters consumed her remains. Her power was divided among them and, over time, split into nine separate inheritable Titan powers, the Nine Titans. The people descended from her, the Subjects of Ymir (Eldians), are the only people who can ever carry a Titan power or be turned into a Titan. Each Titan holder after Ymir lives only thirteen years after acquiring the power. This is called the Curse of Ymir.
The Nine Titans destroyed the nation of Marley and conquered the continent, and the Eldian Empire endured for about 1,700 years. Nobles held the Titan powers within their families. The Fritz family kept the Founding Titan, and the Tybur family kept the War Hammer Titan. The Founding Titan kept peace between the noble houses, which were otherwise in constant conflict.
In Year 743, during the Great Titan War, the 145th king, Karl Fritz, who felt deep shame at the conduct of his people and sympathy for Marley, conspired with the Tybur family to end the empire. Together they invented the story of Helos, a Marleyan hero who defeated the Fritz king, and so incited a civil war with Marleyan help. Eldia collapsed from within. Marley seized seven of the Nine Titans; only the Founding Titan and the Attack Titan escaped Marley's grasp. The Tybur family was allowed to keep the War Hammer Titan for having been first to fight for Marley. Marley distributed the other six Titans among Eldians loyal to it, who were called Warriors.
Karl Fritz withdrew with part of his people to Paradis Island. Using the Founding Titan, he created millions of Colossal Titans and hardened them into three concentric walls, Maria, Rose, and Sina, named for Ymir's daughters. He swore that if the Walls were ever attacked, the Titans within them would be released to trample the world. He then erased the memories of the Eldians on the island and replaced them with a false history, and he bound his descendants to a vow of renouncing war. A few minority peoples, among them the Ackerman clan and the Eastern lineage later associated with Hizuru, were immune to the memory alteration and kept their memories. The Fritz line afterward lived in the Walls under the name Reiss, with the Founding Titan passed down through that family.
Eldians who stayed on the continent after the war were confined by Marley to internment zones and regarded as a lower class. One of these zones was in the city of Liberio. In Year 830 Marley announced a Warrior program, offering that Eldian children aged five to seven could be selected to inherit the six Titans Marley controlled, and in exchange the child and the child's family would receive the status of honorary Marleyans. Selected children trained as Warrior candidates. Warriors wore red armbands, candidates yellow, and ordinary Eldians white. A Warrior kept the Titan for a thirteen-year term and then passed it to a candidate. The passing was done by turning the candidate into a Pure Titan with Titan spinal fluid and having the candidate eat the outgoing Warrior.
The Eldian Restorationists were an underground movement among Eldians seeking to reclaim Eldia's place. A man named Grisha Yeager, a Restorationist, saw the Warrior program as a way to place an agent inside Marley's military, and he sent his young son Zeke into the program. Zeke became the Beast Titan's inheritor. Grisha later joined with a Restorationist named Eren Kruger, who held the Attack Titan. Kruger passed the Attack Titan to Grisha and sent him to the Walls to take the Founding Titan from the Reiss family. Between Years 842 and 843 the next generation of Warriors received their Titans, among them Reiner Braun (Armored Titan), Bertolt Hoover (Colossal Titan), Annie Leonhart (Female Titan), Marcel Galliard (Jaw Titan), and Pieck Finger (Cart Titan).
In Year 845, Marley sent four Warriors to the Walls: Reiner, Bertolt, Annie, and Marcel. Their mission was to destroy Wall Maria's defenders, take the Founding Titan from the royal family, and end what Marley regarded as the cursed history of the people of the Walls. Marley had also sent Grisha, along with an Eldian agent named Eren Kruger who held the Attack Titan, to Paradis ahead of them to take the Founding Titan from the Reiss line. Kruger passed the Attack Titan to Grisha, who then went inside the Walls. Between Years 832 and 835 Grisha had married Carla and settled as a doctor in Shiganshina District, and in Year 835 their son Eren was born.
On the day of the attack, Grisha, now the Attack Titan's holder, killed most of the Reiss family and consumed Frieda Reiss, who then held the Founding Titan, thereby acquiring that power as well. As a man without royal blood he could not use the Founding Titan's full abilities. Grisha then fed himself to his own son Eren, passing both the Attack Titan and the Founding Titan to him. In the same hours, Bertolt as the Colossal Titan kicked open the gate of Shiganshina, and Reiner as the Armored Titan broke the inner gate of Wall Maria. Titans entered Wall Maria and Shiganshina fell. Marcel was eaten by a Pure Titan on Paradis Island, a Pure Titan who then became Ymir, an Eldian woman who inherited the Jaw Titan from the encounter. The remaining three Warriors, Reiner, Bertolt, and Annie, concealed themselves among the refugees.
The outcome is the central hidden condition of Season 1. By the end of Year 845, Eren Yeager, a ten-year-old boy, carries both the Attack Titan and the Founding Titan without knowing it. The Reiss family has been reduced to a hidden remnant. Marley has lost its immediate path to the Founding Titan and has three Warriors on the island.