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Oath in the Bamboo Thicket (竹林の誓い Chikurin no Chikai?) is the 2nd chapter of the 6th volume and the 19th chapter overall of the Attack on Titan: Before the Fall manga, written by Ryō Suzukaze and illustrated by Satoshi Shiki.
In Jorge's story, Sorum sacrifices himself by detonating Angel's bomb in-hand. The explosion is so powerful Mammon's head falls off. Unlike previous attempts, Mammon finally dies. Sorum is the first human to kill a Titan, however, the official report shrouds the true story. Regardless, Sorum's death holds meaning to the corps because he had struck the Titans' weak spot in order for Mammon to die. Under regular circumstances it would be impossible to reach a Titan's neck, however, Angel invents the Device that can make such acts possible. The government does not listen to their ideas and considers disbanding the Survey Corps. However, before that, the corps decide to hold an unauthorized expedition to prove that Titans can be killed. It is Angel who eventually discovers the Titan's weak spot at the nape of the neck, but he disappears. Jorge is given the title instead, and he reluctantly accepts it. Jorge wraps up his story and expresses hope for Kuklo, and departs. He warns them that they should not leave the city for another three months.
Kuklo and Cardina rest for the night and Sharle comes in with tea. Cardina leaves them alone and Sharle confronts Kuklo with a matter that has been bothering her. Kuklo admits that he is unsure how to react at the death of a family member - Sharle hears this and reassures him that she can be his family. Elsewhere, Xavi Inocencio gives his report.
In the final moments of the battle, Jorge is recounting to his comrades, Jorge charges at Mammon with full speed as Sorum Humé prepares to ignite the bomb to decapitate the Titan. Angel Aaltonen cries out in terror as Sorum's bomb engulfs Mammon's head in a fiery explosion. Mammon's head falls to the ground, and a wave of steam bursts out from the Titan's body. Jorge begins to worry that the Titan is regenerating itself, but when the steam settles Angel and Jorge are shocked to see that nobody was left behind from the fallen Titan. Jorge's story ends as they stand dumbfounded, wondering what just happened.
Fifteen years later in the iron bamboo forest, Jorge states that out of the 60 men that embarked on that expedition, 18 returned with nothing to show for their efforts. He says that due to their failure, further expeditions were prohibited by the Royal Government. Kuklo states that there was, in fact, something to show for their efforts, which Jorge agrees to. Jorge states that what they saw was the death of a Titan. Cardina speculates that the mission reports were covered up so that the conservatives like his father could gain political influence.
Kuklo says that there must have been more to the battle. Jorge says that Sorum's bomb must have destroyed the Titan's weak point, one that is difficult to reach with guns or cannon fire. Kuklo realizes that this event led to the creation of "the device." Xenophon notes the genius of Angel's idea, saying that it was the death of Corina that first led to his desire to invent such a weapon.
Jorge explains that even though Angel's equipment and Xenophon's iron bamboo swords were in Survey Corps possession at the time, they were useless while the expeditions had been banned. Because of this, they had to prove that the Titans could be killed before an order was given to seal the gates permanently. This led to an unauthorized Survey Corps expedition. Kuklo is unfamiliar with this significance, which Cardina explains for him. With 27 soldiers, Angel Aaltonen and the Survey Corps succeeded in killing a Titan with the device, locating the weak point at the back of the neck.
Jorge states that Angel's eyesight was damaged heavily in the battle, and Jorge was labeled as a hero. Jorge detested this, since Angel and Sorum were the true Titan-slayers, but he accepted the title since it would boost the reputation of the Survey Corps. Jorge says that after fifteen years, Kuklo has proved to be the perfect individual for using the newer version of Angel's device to kill the Titans. Kuklo declares that he will carry on Angel's determination. Sharle offers her support as a craftsman in aiding Kuklo's cause, and Cardina pledges his support as well. Xenophon agrees to improve Angel's device further while tutoring Sharle. They then return to their homes in the Industrial City.
Afterward, Jorge commands Kuklo and Cardina to continue in their basic training. Kuklo states that he will test the device in the process. Xenophon entrusts Kuklo as his live test subject for prototypes of the device, causing Sharle to worry over Kuklo's safety. Jorge departs from them, warning that they should not leave the Industrial City for at least three months due to Military Police activity.
Later that night, Kuklo and Cardina discuss the gravity of what they learned that day. Cardina makes plans to write up a training menu for them both as Sharle enters the room with tea. Cardina leaves in a hurry, claiming that Xenophon needs him for something. Alone, Kuklo and Sharle have tea together. Sharle asks why Kuklo was so calm when the topic of the Titan that ate his mother came up. Kuklo says that since he never knew his mother or father, he feels no real emotions towards their deaths despite their relation. He recalls the time when he was chained in the possession of Xavi and the way Xavi's father looked at his son with a warmth Kuklo could not understand. As Kuklo wonders if his family would have looked at him the same way, Sharle embraces him, saying that she will be his family. Kuklo is comforted by the warm compassion of Sharle, thinking to himself that this is the warmth he must protect from the Titans.
In the headquarters of the Training Corps, Xavi Inocencio reports to his instructor.