|I see...it seems he did love his second son. Enough to put his mission from the Restorationists aside. It's almost nothing like what happened with me. It does seem he'd learn from his "past" mistakes.|
|— Zeke sees Grisha's change of heart|
After being decapitated, Eren Yeager wakes up in the coordinate, where all the paths converge. Zeke tells him this is possible because he was able to catch Eren's head in his hand before Eren completely died. He tries to get Eren to command Ymir to sterilize the Eldian race, but Eren refuses, saying that he only pretended to go along with Zeke's plan in order to end up here.
However, Zeke is not bound by the vow to renounce war, and Ymir only responds to someone of royal blood. Eren is just a key so Zeke can unlock the Founding Titan's power. Though disappointed in Eren, Zeke chooses to blame their father Grisha for brainwashing him and resolves to save Eren by forcing him to witness their father's memories. Contrary to Zeke's expectations though, he discovers that Grisha was willing to set aside his mission to retake the Founder in favor of staying with his family. Not only that, but Zeke learns that Grisha never stopped thinking about him.
In the middle of the battle in Shiganshina District, Eren Yeager's decapitated head lands in the outstretched hand of his brother, Zeke. This causes Eren to wake up in the desert realm accessible via the paths, where he sees a pillar of light that branches out across the heavens. There he meets an older version of Zeke who sits chained on the sandy ground.
Zeke informs Eren that this is the coordinate, where all paths converge. He believes that this realm is where members of the royal family came whenever they used the Founding Titan's power. Zeke is uncertain how long he has been waiting for Eren to awaken, but feels as though it has been years. Though Gabi Braun succeeded in blowing Eren's head off, Zeke was able to make contact with him in the real world before Eren's life ended, allowing the two of them to succeed in gaining the Founder's power.
A girl in a ragged dress approaches them and Zeke calls her Ymir, the Founder, because he does not believe anyone else would be wandering this place. He believes she creates all the Titans here, where she is allowed an endless amount of time, before the Titans are sent to the physical world.
When Eren inquires about the chains Zeke is shackled by, Zeke explains that they represent the vow to renounce war and thus Eren is the only one who can currently walk freely within the desert realm and, as such, command the Founder. Zeke pleads with Eren to go ahead and have Ymir rebuild Eldians so that they can no longer have children. However, Eren refuses, revealing that he had only pretended to go along with Zeke's plan so that he could end up here, in the coordinate.
Severely disheartened, Zeke asks Eren while he would do such a thing, and Eren replies that it is because he was born into this world. Eren asks Ymir to lend him her strength, but she ignores him and kneels before Zeke.
Zeke laments Eren's mentality as brainwashing by their father, Grisha, but stands up, removing the chains around his body. He explains that he learned many things while waiting for Eren to wake up, and that he can create anything in this land as long as he with his royal blood wills it. Unlike the past members of royalty who accessed it, Zeke entered the coordinate without being tainted by Karl Fritz's ideology, and thus he has learned how to nullify the vow to renounce war. Though Ymir is powerful, in actuality, she simply submits to anyone with royal blood. While they are here, Zeke has the power of the Founder and Eren was simply the key that let him in.
Chains appear around Eren as Zeke explains that he is glad to hear how his younger brother truly feels, but he still does not blame him. Zeke had Tom Ksaver as a surrogate father who rescued him from Grisha. He believes Eren needs someone to rescue him as well, and thus Zeke plans to use the power of the Founder to "fix" Eren.
He does this by forcing the two of them into Grisha's memories, where they see Grisha and Carla Yeager playing with the newborn Eren. Zeke plans to show Eren how his father had sown the seeds of nationalism into him, and he is clearly dissatisfied with how Grisha enjoyed life with his new family as if his old one had never existed.
Zeke shows Eren how his father used his position as a doctor to treat a government minister and get close to those in power. Even though things would go horribly for his family if he was ever found out, Grisha still took the risks. He even found the basement beneath the chapel where the Reiss family would hold their succession rituals. However, he did this years before the day Wall Maria was breached.
They see Grisha ultimately went back to his family, and Zeke concedes that Grisha did seem to love them enough that he set aside his mission to retake the Founder. He muses that perhaps Grisha had learned from his past mistakes, but Zeke brushes that aside because now Eren is doing exactly as Grisha would have.
The two of them stand in the Yeager family basement and talk near a sleeping Grisha when their father stirred, apologizing to Zeke. He looked up in the direction where Zeke stands by a cabinet. Both Eren and Zeke are surprised that Grisha in his memories seemed to be able to see Zeke, asking if he was there. Confused, Grisha sat back down and dismissed the vision as a dream, because he could not imagine why a bearded old man would be his son.
Zeke regards his father with disbelief as Eren reaches for the door to the basement, prompting Zeke for the next memory.