Swear Upon Wall Maria

Wōru Maria ni Chikau (ウォール・マリアに誓う) is an extra chapter included in the 6th volume of the Attack on Titan: Before the Fall manga, written by Ryō Suzukaze and illustrated by Satoshi Shiki.

Summary
Fifteen years ago, Angel Aaltonen looks up to Wall Maria with determination. He climbs a ladder to its summit, seeing Garrison soldier Maria Carlstead there on lookout. Without turning to greet him, she says that when her husband Sorum Humé died, she was waiting for his return from the expedition one week ago. Smiling with tears in her eyes, she asks Angel if Sorum was heroic. Angel looks away, thinking back to a time when they were all together.

Some time ago, Angel tells Maria and Sorum that he plans on joining the Survey Corps expedition to study the Titans. Maria objects to his wishes, but Sorum drunkenly assures Angel that he will keep him safe. Angel says that he has already received permission from Jorge Pikale to join the expedition. As Maria sighs worriedly, Sorum thanks Maria for protecting their homes. He goes on to say that he will transfer out of the Survey Corps after the upcoming expedition for the sake of the baby that Maria is now pregnant with.

On top of Wall Maria, Angel says that Sorum gave his life to save his own. Maria interrupts him, declaring that she is certain Sorum was heroic. Angel asks Maria what she plans to do, to which Maria says that she will stay in the Garrison and protect her home. Maria asks Angel of what he is planning, and Angel says that he must go beyond the walls again. On the verge of tears, he declares that he must develop a weapon to put an end to the idea of the invincibility of the Titans, saying that it is the only way he can repay Sorum's sacrifice.