Attack on Titan season 3 episode 11 Bystander
It is two months since Historia's coronation and the military have purged remaining traces of the old regime. This cost talented manpower, however they gained an abundant supply of hardened Titan crystals from the Reiss estate. Meanwhile, Eren perfects his Titan hardening ability and Hange develops a new weapon called the "Executioner from Hell" that allows defenders to easily kill Titans who attack the walls without loss of human life. Eren recalls that Cadet Corps Commander Keith Shadis was the man in his vision who knew his father and the Scouts visit him. Keith tells them that he met Eren's father Grisha twenty years ago outside Wall Maria, suffering from amnesia. Grisha learned about life within the walls, offered his abilities as a doctor, and later married Carla who gave birth to their son Eren. During the Fall of Shiganshina, after learning about his wife's death, Grisha took Eren into the woods where Keith later found the boy wearing a key around his neck. Keith resigned as Commander of the Scouts due to his lack of confidence and the loss of his men, and instead focused on training the Cadet Corps. After hearing his story, the scouts make preparations to retake Wall Maria.