The Founding Titan is the first Titan to have existed and is the progenitor of the other eight intelligent Titans that we see in Attack on Titan. The Founding Titan has the powers to control the other Titans at will and rule over them, making it a powerful weapon sought by both Marley and Eldia. The Founding Titan is also referred to as the Coordinate, a point where the ‘paths’, an invisible connection between all the Eldians and Titans, cross and converge.
Eren Yeager inherited the Attack Titan and the Founding Titan from his father Grisha Yeager during the events that followed the fall of Shiganshina district. But, even though Eren possesses the founding Titan, he can’t use its powers freely.
When can Eren use the powers of Founding Titan?
The complete extent of a Founding Titans powers can only be unlocked by a person with the royal bloodline (Eldian), i.e the person should belong to the Fritz or Reiss family. Since Eren does not have the royal blood in him, one of the following three conditions need to be fulfilled for him to be able to use the powers of the Founding Titan. Note that Eren need not be in his Titan form in any of the following situations, to be able to use the powers of the Founding Titan.
Should be in contact with a Titan with royal blood:
Eren can use the powers of the Founding Titan if he is in physical contact with a Titan that has the royal blood. The Titan can be a pure Titan or an intelligent Titan, it doesn’t matter.
Eren is able to use the Founding Titan’s powers to control the other Titans when he punches the palm of the Pure Titan that was about to devour him and Mikasa. This was the same Titan which ate his mom when Shiganshina district was breached. In the brief moment that he establishes physical contact with this Titan, his emotions and will to stop that Titan get communicated to the other pure Titans as orders, who then go on to devour it.
However, the reason why he was able to use the Founding Titan’s powers only becomes clear in Season 3. The pure Titan that Eren punched was none other than Dina Fritz. Dina Fritz was an Eldian who possessed the royal blood. She was the first wife of Grisha Yeager (while in Marley) and a member of the Eldian restorationists. She was converted into an Abnormal Titan for committing treason against the Marley Empire.
When Eren punched Dina Fritz’s Titan, he unknowingly came in physical contact with a Titan with royal blood, thus being able to activate the powers of the Founding Titan.
This is the reason why Eren manages to control Titans but only sometimes. This reason is further solidified in Season 3 Episode 9, when Eren tries to use the powers of the Founding Titan on Rod Reiss’s pure Titan, but isn’t able to do so. This was due to the fact that he was not in contact with a Titan with royal blood at that moment.
Should be in contact with human who possess Royal blood:
It is revealed that Eren can activate at least some parts of the Founding Titan’s powers when he is in contact with an Eldian having royal blood. In Attack on Titan season 3, when Eren touches Historia and Rod Reiss on multiple occasions, it triggers some parts of memories that belonged to the previous inheritors of the Titans.
This is clearly evident in the scene where Eren touched Historia’s hand during her coronation and became speechless for a moment, after it triggered a particular memory which has not yet been revealed in the anime. These memories which surface are quite unpredictable and there is no way that Eren can exert control over them. It is not known if Eren can control the Titans by simply being in contact with a human (and not a Titan) that possesses the royal blood.
The following part of the post contains spoilers for the chapters from Attack on Titan manga which have not yet been covered in the anime adaptation. If you are not up to date with the content of the series, we request you read further at your own discretion!
By being in Contact with a Titan shifter with royal blood (dicey dicey scenario):
When a titan shifter with royal blood comes in contact with Eren (who possesses the Founding Titan but lacks the Royal blood), then both the Titan Shifter and Eren can access the powers of the Founding Titan completely. However, this might always not hold true (at least not before Ymir Fritz was freed from her submissive state in the coordinate), as the titan shifter with royal blood has the preference in this scenario.
When Zeke Yeager came in contact with Eren, it was Zeke who was able to use the complete extent of the Titan’s powers and not Eren as the latter lacked the royal blood. However, after Eren frees Ymir from her millenia of submission to the Fritz king, she complied with his wishes. Meaning, after this encounter, Eren no longer needs the royal bloodline to command the complete powers of the Founding Titan.
Also, Eren is not bound by the vow of Karl Fritz, who renounced war and prevented the Founding Titan from being used to devastate the world again.