Eren wakes up in the strange blue cave, a place he’s never been, but which he still finds familiar. Rob appears with Historia and tells Eren there’s a reason for that. Then Rob and Tori place their hands on his back, and the memories come back fast and furious. Not those of Eren himself, but those of his father, and how he came to inject his son with the royal power of Titans.
The memory awakening works both ways, with Historia suddenly remembering her half-sister Freida, who would periodically visit her to make sure she was doing okay, each time using her Titan “Scream”-derived power to wipe all of Tori’s memories before departing.
Rob completes the puzzle by combining Historia’s memories of Freida with those from Eren’s father Grisha’s POV, while also explaining the severe damage to the chapel above. Grisha came to steal the Titan powers from the Reiss’, specifically Freida, and while her Titan form was the most powerful, she lacked Grisha’s combat experience, and he ate her, thus stealing her Titan-controlling “Scream.”
Grisha then passed “Scream” to Eren via injection. To ensure no other Reiss would have the power, the Grisha Titan slaughtered Rob’s entire family. Only Historia wasn’t there, and she still carries the noble blood. Now that Rob has Eren, he no doubt intends to recover “Scream” for Historia. Whether she understands that will mean the end for Eren, I can’t yet say.
Back in the capital, Pyxis tells Erwin their coup is all for nothing if Rob Reiss obtains Eren’s “Scream.” They may have toppled the fake king, but they seem reluctant to let another take up the mantle of leadership so soon…not if they’re not sure it’s what’s best for humanity, which is all Pyxis cares about, and the only reason he sided with (the very sadistic) Zachary and Erwin.
Rob trusts Kenny, but he probably shouldn’t, if Kenny’s smirk and scoff upon turning his back on Rob is any indication. Not to mention the fact Kenny is an Ackerman, a clan of warriors who once served as the swords and protectors of kings.
They started to become persecuted along with the Asian races, both groups that were immune to the royal power to wipe memories. We learn this in a flashback in which Kenny talks with his grandfather who is on his death bed. We also learn of Kenny’s sister Kuchel, who was pregnant with (I’m guessing) Levi at the time.
In any case, if the goal is to save Eren from being eaten, Mikasa, Armin, Levi, Hange, Sasha, Connie, and Jean will have to get through Kenny and his slightly depleted but still formidable Anti-Personnel Control Squad.