The Founding Titan was one of the Nine Titans and the first of all Titans. Its Scream could create and control other Titans and modify the memories and body compositions of the Subjects of Ymir, but this power had historically only been able to be used by members of the royal family.
According to Marley's Titan Biology Research Society, the Founding Titan was the point where the paths connecting all Subjects of Ymir and Titans cross. Because of this, those who hail from Marley sometimes called it the "Coordinate".
Abilities: The Founding Titan had several abilities that were activated by screaming. The inheritor of the Founding Titan did not have to transform in order to use these abilities; human screams would suffice. Furthermore, the screams did not have to be loud, or even "screams" in the truest sense: Frieda Reiss used the memory-alteration ability by casually talking, although her target was only Historia, who was right in front of her.
History: Around 2,000 years ago, Ymir Fritz's village was attacked and enslaved by the Eldians, who then cut out their tongues. One day after a pig was released, the tribe's ruler, Fritz, called all of the slaves and asked for the culprit. The slaves all began to point at Ymir, who was then "freed" only to be hunted by dogs and men.
Story: In the year 845, 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 Kruger with taking the Founding Titan thirteen years before.
Trivia: The Founding Titan was the only one of the Nine Titans that had a bloodline restriction, as only royals could freely use its abilities. Interestingly, when Zeke Yeager (a descendant of the Fritz family) inherited the Beast Titan, he gained abilities similar to the Founding Titan. Eldian artwork had on several occasions depicted the Founding Titan with a .
Source: Founding Titan, available under CC BY-SA 3.0.