Illumination in the Dead of Night (夜陰の明徴 Yain no Meichō?) is the 4th and final chapter of the 15th volume and the 56th chapter overall of the Attack on Titan: Before the Fall manga, written by Ryō Suzukaze and illustrated by Satoshi Shiki.
Overview
Xavi reveals the true culprits behind the recent equipment sabotage which turns out to be the group that heckled Rosa early on in the training program. Carlo calls a meeting to discuss this latest discovery. Jorge concludes it was done out of jealousy that the other trainees felt toward the specialized training group. Later at night, in the guardhouse, Felix reveals to the other trainees his distress when he had failed the first two tests, as an explanation for the sabotage. Meanwhile, Kuklo goes and properly thanks Xavi and Fuchs for their efforts. Xavi flatly states it will not happen again and that now, Kuklo is in debt to Xavi and will owe him a favor in return.
Summary
The trainees recognize the culprits behind the sabotage
After Xavi easily subdues Felix, he chastises Kuklo for being tardy; as the other trainees look at Kuklo for clarification, Xavi motions to a corner of the room. There, he points out the true perpetrators who are tied up and unconscious; after Kuklo thanks Xavi for his help, the latter merely mentions that he gave the order. He then belittles Felix and leaves. Kuklo goes over to the people in the corner and the trainees recognize them as the ones who mocked Rosa's performance during the initial stages of the training course.
Later, Carlo assembles them to the meeting room to discuss the recent findings. Kuklo informs them that before he left, Angel divulged to him that the rods jammed into the sets of equipment were from the tip of a knife or dagger; thus, the initial assumption of the saboteurs' knowledge of the vertical maneuvering equipment was incorrect. Both Kuklo and Sharle worked together to convince Xavi to assist in their efforts to track down the culprits. Kuklo goes on to mention Fuchs, Xavi's spy, who he sensed eavesdropping earlier and how he helped assist them as well.
Jorge dubs the incident as an act of jealousy
Jorge mentions that he interrogated the culprits and apologizes for failing to sense the air of unhappiness the general trainees had towards the specialized training group. He finishes that the trainees believed that the training group had essentially been accepted into the Survey Corps due to passing the tests and it was done out of jealousy. Cardina proposes to Carlo they merge all the trainees together. Jorge proposes that Carlo announce the stakes of the upcoming expedition to all the trainees. Momentarily taken back by this, Carlo decides to reconvene after Angel arrives back to the training grounds.
Felix recalls being dejected on failing the second test
As Kai and Ivo complain about being forced to stay in the guardhouse as punishment for their actions, Felix mentions it is a deserved punishment. Kai asks aloud if the earlier fight between them and the culprits was the instigating factor in the sabotage efforts; Ivo recalls Kuklo's explanation of the perceived special treatment the specialized trainees were given and this caused the others to feel inferior in their own abilities. Despite not agreeing with their actions, Felix agrees due to harboring similar feelings after he failed the initial and second testing periods.
That night, Kuklo goes to Xavi and once again thanks him and Fuchs for their efforts. Not looking up, Xavi mentions it was merely due to Sharle being involved and that it will not happen again. As Kuklo turns to leave, Xavi states that he did Kuklo a favor and that soon, he will have to do the same; slightly worried about what it could be, Kuklo departs.