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 with the ability that is capable of allowing the user 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, which is also referred to as the "Founding Titan". It is currently in the possession of Eren Yeager.

Titan control
The Founding Titan grants the user the ability to control Titans at will and make them follow virtually any order. This is seen when Eren Yeager unintentionally commanded nearby Titans to eat the Dina Yeager's Titan and to attack the Armored Titan. It was also used by the First King to build the Walls, making many Colossus Titans 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 the first of all Titans, to use the Titans to accomplish great deeds for her subjects, the forefathers of the nation of Eldia. The real limit of this power is unknown, but as it can be observed, it extends to the point of making Titans perform actions that threaten their own lives. It was also mentioned by Rod Reiss that the power is so strong, it has the potential to wipe out all the Titans if the user is able to wield it to its full strength.

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 they are skilled enough. It was used by Frieda Reiss to remove Historia's memories of her visits, and by the First King to make humanity forget all the history of the world before the building of the Walls, becoming the only holder of the memories of the world. However, it is known that the minority bloodlines of the Walls 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. This caused the persecution of the former two at the hands of the Royal Government. 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 in the nation of Eldia while the eight factions of the Titans warred against each other.

Memory inheritance
Founding Titans are able to inherit the memories of the previous holders of the Founding Titan. This ability goes back to the memories of the First King, and perhaps as far back as Ymir Fritz herself, in a time before the building of the Walls, giving each new holder omniscient-like knowledge about the world, including the reason the Titans exist and how the Walls were created. Nevertheless, it seems that only the Reiss bloodline can activate this ability from the beginning, since Uri and Frieda Reiss showed signs of being aware of that knowledge immediately after obtaining the Founding Titan, while Eren was only able to see some vague memories of Frieda after years of having received it. The user is referred to as the living compendium of humanity because of this.

The 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 has been seen when Eren unleashed the Founding Titan after 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 Reiss, this method did not work. Eren has considered whether touching a royal-blooded Titan will work a second time, though he has not spoken this idea publicly out of concern for Historia's safety. Despite inheriting the memories of the world and the power to control the Titans, no Founding Titan 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 the power. This was because these Founding Titans had inherited the ideology of the First King as well as his memories. Frieda used to suffer from fits of madness and depression due to those memories, and claimed that humans 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 between the eight subservient Titan houses, 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 to no use of the Founding Titan for the next three years, until the fall of Wall Maria. 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 Kruger 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 Kruger, 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 Kruger'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.

Trivia

 * The Founding Titan, as well as other Titan abilities, is retained in the user's spinal cord fluids, so it is not necessary to actually eat the whole wielder to obtain the power.
 * 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.