The Ending Of Attack On Titan Season 3 Explained

"Attack on Titan" first set up the mystery of Grisha Jeager's basement all the way back in its first season. To be fair, after so much time has passed, you would almost be forgiven for forgetting it ever existed. Nonetheless, by the end of Season 3, now that the Scout Regiment has successfully overthrown the Eldian government and installed Historia as Queen, defeated the three Titan-Shifters, and reclaimed Shiganshina, they can finally unveil the truth behind it all. In Eren's basement, the group finds a desk containing a photograph of Grisha with an unfamiliar woman and child.

Grisha's writings reveal the truth about the world outside the walls, how there are still thriving civilizations with advanced technology (such as photography). Later, we see Grisha's memories through Eren's dreams, revealing his upbringing living in a country named Marley, where he and other Eldians are persecuted by the government. Supposedly, millennia ago, an Eldian named Ymir made a deal with the Devil of All Earth to become the first Titan. When she died, her powers were split amongst her descendants, who used their Titan abilities to conquer and oppress Marley. Eventually, the Marleyans revolted and gain control of some of the Titans, overthrowing the Eldians.

The Eldian King then gathered a portion of his population and fled to Paradis Island, where he used his own Titans to create the walls. Since then, Marley has been persecuting their own Eldian population, transforming dissidents into Titans to keep the Paradis Eldians in check.