Grisha's basement

The Yeager house's locked basement: the goal of five years of war. Inside, Grisha's journals and a photograph reveal that humanity did not die out beyond the Walls.

Three books hidden in a false-bottomed drawer tell everything: Marley, the Eldian Empire, the Warriors, the internment zones, the fate of shifters who die thirteen years after inheriting their power. In the last one, Grisha explains how he got the Attack Titan from Eren Kruger. The 'world' the Survey Corps died for turns out to be an island surrounded by enemies.