Founding Titan

"Frieda's Titan was the greatest of all the Titans. It possessed an invincible power..."

- Rod on holding the Founding Titan

The Shiso no Kyojin (始祖の巨人) is one of the Nine Titans, granting its user the unique ability to control the actions of the Titans, modify the memories of others in the majority bloodline of the Walls, and inherit the memories of those who previously possessed the ability. While the last true Founding Titan was Frieda Reiss, its power is mostly dormant in the possession of Eren Yeager, usable only under very rare circumstances.

Titan control
The Founding Titan grants its user the ability to control Titans at will and make them follow virtually any order. This was demonstrated by Eren Yeager, who unintentionally commanded nearby Titans to eat Dina Yeager's Titan and attack the Armored Titan. It was also used by the First King to build the Walls, commanding many Colossus Titans to harden their bodies and confine themselves into the resulting structures. Over a thousand years earlier, this same ability was used by Ymir Fritz — the first holder of the Founding Titan and progenitor of all Titans — who utilized the Titans to accomplish great deeds for her subjects, Eldia's forefathers. There is no known limit to the Founding Titan, though its range is so extensive as to make Titans perform actions which threaten their own lives. Rod Reiss stated that the power has the capacity to wipe out all the Titans, if used to its full potential.

Memory manipulation
Those who possess the Founding Titan are able to erase or modify the memory of a single person, or even those of entire countries if skilled enough. It was used by Frieda Reiss to remove Historia's memories of her visits, and by the First King to make his subjects forget the history of the world before the Walls' installation, making him one of the few inside the Walls knowledgeable of the world's true status. However, it is known that the minority bloodlines of Paradis such as the Ackerman bloodline, the Asian clan, and the noble families are immune to this mental manipulation due to not sharing a common bloodline with the majority race within the Walls.

Throughout the many centuries before the raising of the Walls, this ability was used by the Fritz family — the ancestors of the Reiss family — to maintain order amongst the eight warring Titans of Eldia.

Multi-generational memory inheritance
While the other Nine Titans are evidently able to inherit the memories of their predecessors to some extent, Founding Titans are able to do so with absolute clarity for multiple generations. In the Reiss family, memories held by this ability trace back to those of the 145th Fritz King, and perhaps as far back as Ymir Fritz herself. This grants each new holder well-nigh omniscient knowledge of the world, including the reason the Titans exist and how the Walls were created. The user is referred to as the living compendium of humanity because of this.

Nevertheless, only the royal bloodline can immediately acquire such knowledge; while Uri and Frieda Reiss displayed awareness of the aforementioned soon after acquiring the Founding Titan, Eren was only able to see vague memories of Frieda's after years of having received it.

Bloodline necessity
It has been said that only a member of the Reiss family, and by extension the ancient Fritz family, can make full use the Founding Titan. However, it appears that if the Founding Titan is held within someone outside of the royal family, the power can still be used if the holder is in physical contact with a member of the royal family. This was seen when Eren unleashed the Founding Titan upon striking the hand of Dina Fritz's Titan form, and some time later when Rod and Historia Reiss laid hands on Eren's back, triggering the resurfacing of Grisha Yeager's memories. However, when attempting to see past memories while holding hands with Historia, this method failed.

Despite inheriting the memories of the world and the ability to control the Titans, no Founding Titan of royal blood expressed any desire to resolve the Titan threat and free humanity after inheriting the power, even if they had expressed the desire to do so before receiving it. This was because these Founding Titans inherited the ideology of the First King along with his memories. Frieda used to suffer from fits of madness and depression due to those memories, and claimed that Eldians were sinners who deserved their punishment.

History
Nearly 2,000 years ago, Ymir Fritz gained the power and became the first of all Titans. While Ymir is said to have been the progenitor of all Nine Titans, her Titan is only ever attributed by name to the Founding Titan. After holding her power for thirteen years, Ymir died by unknown means, and her power was split between nine inheritors, the ancestors of the Nine Titans. For the next 1,700 years, Ymir's Founding Titan presumably remained within her direct descendants in the royal Fritz family. The Eldian Empire, founded by the Nine Titans after Ymir's death, possessed a history of strife for control over the Titans between the various familial houses subservient to the royal family, but order was nonetheless maintained thanks to the Founding Titan.

After these seventeen centuries, the Founding Titan was inherited by the 145th Fritz monarch. King Fritz chose to abandon the conflicts of Eldia and relocated the capital of Eldia to the island of Paradis where he used the Founding Titan to guide many Colossus Titans in the building of three Walls: Maria, Rose, and Sheena. In the absence of the Founding Titan, Marley waged war against Eldia and stole seven of the Nine Titans. Over the course of the war, Eldia lost all its territory on the continental mainland and its people were forced to flee overseas to Paradis where they entered into the Walls. Once the people were gathered within the Walls, King Fritz made use of the Founding Titan to erase the memories of the outside world from the minds of the people, leading them to believe they were the last remnants of humanity in a world overrun by the Titans. Upon the closing of the gates of Wall Maria, King Fritz announced to his relatives who chose to remain across the sea that a pact had been made with the Founding Titan. In order to secure peace for Eldia while simultaneously arranging for the nation's eventual destruction, King Fritz ensured that all successors after him would be overcome by his will if they chose to use the Founding Titan for Eldia's sake. Some time afterwards, the King passed his power over to his descendants in the royal family, now known as the Reiss family.

For generations, the Reiss family passed the Founding Titan down from successor to successor. Eventually, the Founding Titan was inherited by Uri Reiss from his father. Early in his life with the Founder, Uri protected himself from Kenny Ackerman using his Titan, and he would later become a close friend of his would-be assassin. Some time later, in the year 842, Uri Reiss passed on the Founding Titan to his niece Frieda Reiss, daughter of his older brother Rod Reiss. Frieda presumably made little use of the Founding Titan for the next three years, though on occasion she visited her half-sister Historia Reiss and used the Founding Titan to erase her memories of each visit. When three years had passed, the fall of Wall Maria began. After hearing of the attack on Shiganshina District, the Reiss family gathered in the underground cavern beneath the chapel where the Founding Titan had been passed on for generations. While in the cavern, they were visited by Grisha Yeager, who had been tasked by Eren Krueger with taking the Founding Titan thirteen years ago. Grisha identified himself as an Eldian and Subject of Ymir and begged for Frieda to use the Founding Titan to save the people of the Walls, but his pleas went unanswered. Seeing no choice but to fulfill his promise to Krueger, Grisha transformed into the Attack Titan and fought with Frieda's Founding Titan. Due to her inexperience, Frieda was quickly overpowered and Grisha stole her power, killing Frieda before turning his attention to the Reiss family and wiping out their line, save for Rod who escaped with his life.

Grisha returned to Wall Rose shortly afterward and searched for his family. He found his son Eren and adopted daughter Mikasa, but learned that his wife Carla had been killed. Grisha entrusted Krueger's mission to Eren, leading him into the woods alone and injecting him with a Titan serum. Eren would have no memory of the event for the next five years, and as a mindless Titan he consumed his father, inheriting both the Attack Titan and the Founding Titan.

Clash of the Titans arc
For five years, the power of the Founding Titan remains unused until the year 850. Believing that Eren Yeager is the current holder of the Founding Titan, the "Coordinate" sought after by Marley, the warriors capture him with the intent of taking him to their "hometown". At this time, the Survey Corps comes to his rescue and a chase ensues. When Eren is freed from the warriors' grasp, he comes into contact with the Titan of Dina Fritz during the Wall Rose invasion, his father's first wife and a descendant of royalty. In a moment of rage, he punches the hand of Dina's Titan, contacting her royal matter, and the power of the Founding Titan comes forth in that moment in the presence of her royalty. Following Eren's will, all the Titans nearby attack Dina's Titan, tearing it to pieces and allowing for the Survey Corps to escape. When Reiner Braun attempts to pursue Eren, he threatens him and the other warriors to stay away, and the Founding Titan reaches out to the Titans nearby, causing them to chase after Reiner's Armored Titan.

The Uprising arc
Eren's ability to control the Titans becomes a key mystery for the Survey Corps. Over a week after the Wall Rose crisis, the Founding Titan begins to leak memories of its previous holders into Eren's mind, where he sees visions of Frieda Reiss before forgetting the memories. Around the same time, Historia Reiss begins seeing memories of Frieda's visits in her dreams, though she does not remember them when she awakens.

Some time later, Eren and Historia are captured by Rod Reiss, who desires to feed Eren to a Titan Historia in order to return the Founding Titan to the Reiss family. In the underground of the Reiss Chapel, Rod and Historia place hands on Eren's body, and their royal presence brings out the power of the Founding Titan once more, and Eren sees the memories of his father Grisha taking the Founding Titan from Frieda five years before. Rod recalls the story of the event and explains the nature of the Founding Titan to Historia and how it influenced the monarchs of the Reiss family since the time of the First King. Historia nearly accepts the task, but refuses it in the end. In desperation, Rod ingests the Titan serum meant for Historia, becoming a Titan of gargantuan size. After escaping from the underground of the chapel, Eren attempts to use the power of the Founding Titan to stop Rod's Titan from advancing towards Orvud District, though he makes no progress and does not understand why.

Return to Shiganshina arc
Two months later, in the aftermath of the battle of Shiganshina District, the Survey Corps comes to the basement of the Yeager family household where it is said that Grisha had kept the secrets of the Titans. In one of three journals kept hidden within the basement, they read Grisha's story of his childhood when he learned the nature of the Founding Titan during his upbringing. In Grisha's past, he had joined a group of Eldia Restorationists who desired to take the Founding Titan from the king of the Walls in order to restore Eldia to glory, though the organization as a whole failed. Grisha alone was given the task of taking the Founding Titan by Eren Krueger, an Eldian spy in the government of Marley.

When the military of the Walls gathers to discuss the discoveries from the basement, Eren thinks back to the time when the Founding Titan came forth in the presence of the Titan that ate Eren's mother. Recalling his father's memories of his wife Dina, Eren realizes that Dina's Titan was responsible for the presence of the Founding Titan. However, Eren decides against revealing his discovery to the military, fearing that the idea of turning a royal into a Titan for Eren to come into contact with may put Historia Reiss' life at risk.

Trivia

 * Hange Zoë created the term "Founding Titan" (始祖の巨人) to refer to the ability and any Titan shifter who could master its power. Coincidentally, the Eldia Restorationists and other inhabitants of Marley also referred to the power by this name.
 * Eldian artwork has on several occasions depicted the power of the Founding Titan as a . This may be an artistic choice meant to represent the scream-based trigger which activates the Founding Titan's power to control other Titans.