WARNING: The following contains spoilers for Attack on Titan Season 4, Episode 20, "Memories of the Future," now streaming on Funimation, Hulu and Crunchyroll.
In one of the most stunning twists, Attack on Titan showed how Eren Jaeger has been using the Attack Titan's special ability to manipulate almost everything. In the episode, "Memories of the Future," Grisha Jaeger, Eren's father, revealed that the Attack Titan allows its user to see the memories of the Titan's future inheritors. Not even the royal family knew this future sight power existed. Using this ability, Eren is ultimately responsible for several key events throughout the series' history.
Grisha is "the one who began this story," when he slaughtered the Reiss family, stole the Founding Titan and passed the Founding and Attack Titans onto his son. However, now the audience has been shown that Grisha was acting on orders from a future version of Eren. The Attack Titan has always fought for freedom, refusing to bow to anyone, least of all the Fritz bloodline, and now it's clear that this defiance stems from Eren Jaeger. Everyone who has ever held the Attack Titan has been acting on memories from the future sent by Eren, and each inheritor worked to achieve whatever endgame Eren has shown them.
The Attack Titan's Special Ability, Explained As Simply As Possible
Despite Grisha's words that the Attack Titan can peer into the memories of future inheritors, he later wondered why Eren hadn't shown him everything about the Colossal Titan's attack. This means that whoever presently holds the Attack Titan cannot actively look at the future, they can only see what the future holders show them. It's important to note that Eren himself has not been seeing the future this entire series, and it's either very good or very bad that Eren has seemingly never received a memory from a future inheritor of the Attack Titan.
As Eren and his brother Zeke traversed the memories of Eren's childhood, their father was clearly aware of their presence on multiple occasions. This is not the power of the Founding Titan via Zeke, but Eren's Attack Titan. Grisha is not technically seeing his adult sons, he is seeing Eren's memory of that exact moment that Eren is essentially live-streaming to Grisha via the Attack Titan's ability. This is how Grisha heard Eren's words goading him into killing the Reiss family, Eren was sending his father the memory of his words as he was saying them, effectively allowing Eren to communicate with a ghost from the past.
Eren's Manipulations Are Responsible For Most Of The Series' Events
Bringing Eren into these memories was the worst thing Zeke could have done, because it allowed Eren to play Grisha like a puppet, knowing exactly what he needed to show him to get him to carry out Eren's will. Eren showed the Colossal Titan's attack to Grisha, but not whether Carla and Mikasa survived, forcing Grisha to take future Eren at his word that killing the Reiss family was for the best. Given what the series has shown regarding the war between Paradis and Marley, the ends might justify the means, but Grisha's ominous plea to Zeke that he stop Eren's terrible plan makes it clear that the worst of Eren's manipulations are yet to come.
Left to his own devices, Grisha Jaeger would not have slaughtered the Reiss family, Eren would have never gotten his Titan powers, and the war with Marley would have gone very differently. It's unknown what Eren's endgame is, but he is knowingly moving towards a future dark enough to make his own father turn against him. As he has orchestrated the entire series up to this point and killed innocents and children along the way while having an unclear or obscured plan, Eren Jaeger appears to be on the verge of becoming Attack on Titan's ultimate villain.
