The Founding Titan was one of the Nine Titans and the first of all Titans. Its Scream could create and control other Titans as well as modify the memories and bodies of the Subjects of Ymir, though historically only the royal family could use this power.

According to Marley’s Titan Biology Research Society, the Founding Titan served as the “Coordinate”, the point where the paths of all Subjects of Ymir and Titans intersect.

Eren’s Founding Titan form was known as the Doomsday Titan 

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 Reiss, who was right in front of her. 

The Founding Titan could transform Subjects of Ymir into Titans and could even make them as massive as the Colossal Titan. Karl Fritz presumably used this ability to create the thousands of Colossal Titans that made up the Walls. Zeke Jaeger's Beast Titan could also transform Subjects of Ymir into Titans by screaming after injecting those he wishes to transform with the beast's spinal fluid. For this reason it was said that Zeke's Beast Titan had abilities similar to the Founding Titan.

By screaming, the Founding Titan could control Titans at will and make them follow any order. Eren Jaeger demonstrated this by commanding Pure Titans to attack Dina Fritz's Titan and the Armored Titan. It was mainly effective on Pure Titans, though those with Titan powers could still feel its influence. Karl Fritz used it to build the Walls by commanding Colossal Titans to harden their bodies. Eldia once used this ability to command an army of Titans in war, turning them into weapons of mass destruction. The ability had no known range, and Rod Reiss claimed it could wipe out all Titans if used fully. 

The Founding Titan could modify minds and memories of the Subjects of Ymir, even entire populations. This ability was tied to Titan control, since Titans were transformed humans. Karl Fritz used it to make the Subjects of Ymir inside the Walls forget history before the Walls. Frieda Reiss used it to erase Historia’s memories of her visits. Those not Subjects of Ymir were unaffected, like the Oriental clan and the Ackermann family, who were resistant despite being Subjects of Ymir.

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The Founding Titan was capable of changing the body composition of any Subject of Ymir. An Eldian king from 600 years ago once used this power to make the Subjects of Ymir immune to a rampant epidemic. Zeke Jaeger wanted to use this ability to make all Subjects of Ymir sterile, so they would die out in a generation and rid the world of Titans once and for all. 

The Founding Titan could telepathically connect with all of the Subjects of Ymir by way of the paths they were connected to. Eren used this ability to communicate with every Subject of Ymir when announcing his plans for the Rumbling of the world.