The Demon in Your Heart

"The Demon in Your Heart" (心に棲むオニ) is the title of the third chapter of the Seraph of the End: Vampire Reign manga series, written by Takaya Kagami and illustrated by Yamato Yamamoto.

Synopsis
Shinoa Hīragi demonstrates her cursed gear to Yuichiro Hyakuya before Yoichi Saotome runs towards them. Bullies are pursuing him though this time one of them has not returned from an unauthorised area thus Yu, Shinoa and Yoichi decide to help.

They enter a forbidden chamber containing cursed gear to find the missing boy who has been consumed by a cursed gear weapon. Yu touches the weapon and finds his orphanage family before him saying he left them in order to save himself.

Yu understands that this revenant of Mika with him is not Mika at all. Yu succeeds in breaking the illusionary hold of the demon within by himself.

Within Sanguinem, Ferid Bathory is in fact alive and interacts with someone else thought to have died: Mikaela Hyakuya.

Summary
Soon after the vampire incident in the classroom, Yu is met by the girl whom he rescued, receiving a love letter from her which he promptly disregards. Shinoa watches in on the event, teasing him about being a heart-breaker and a virgin. She goes on to talk about how the Japanese Imperial Demon Army encourages youths to have sex and get pregnant outside of marriage, as the world is badly in need of active repopulation.

Yu tries to steer the conversation in another direction, posing the question of why he hasn't been put on the front lines yet after showing how brave and capable he was in helping to kill that vampire. Shinoa responds by telling him that he's simply not equipped to handle a fight with any vampire, regardless of his training, and further demonstrates by brandishing a pen that transforms into an ethereal scythe known as Shikama Dōji (four-scythe child).

After he tries to steal it from her (and gets utterly spanked for trying), Yoichi shows up with the bullies he was previously trying to get on good terms with, this time with the roles reversed as the bullies come asking Yu for help. Apparently one of their friends had wandered into an unauthorized area and had not returned.

As the three go to investigate, Shinoa explains that the Forbidden Chamber that the bully had wandered into was actually a training facility for the vampire extermination unit. While calling for the active members of the unit for assistance would have been a more pragmatic idea, Shinoa saw this as a chance for Yu and Yoichi to take another step towards making a contract with demons to utilize their own sets of Cursed Gear. She even explains that the entire school is basically a lab rat for grooming candidates for Cursed Gear; grades are of no consequence in choosing suitable wielders of demonic weaponry like Shinoa's or Guren's.

As they step into the chamber, they know to expect the worst for the boy that wandered in. Since he was not authorized by the upper levels of the military to enter, his heart had surely been devoured by a demon. As they made their way in, the room opened up to a vast, seemingly desolate hallway, save for the hundreds of weapons that lay scattered on the ground. Each and every one of them were Cursed Gear, which housed the power of demons within.

The boy they were looking for stood with a battle axe, his mind lost to the weapon. Yu eagerly charges at him headlong and clashes with the boy, trying to disarm him and take the weapon for himself.

Though Shinoa had warned him that simply touching Cursed Gear would leave him also susceptible to possession, Yu tosses his sword aside and grabs the axe anyway. He blacks out and finds himself in a vast white void, with the orphanage children and Mika standing at his feet.

With the nagging, gouging taunts of the children that he abandoned to save himself, Yu was surrounded by every guilty thought that could have ever passed through his mind. However, he picked up pretty promptly that the boy that appeared to be Mika was not actually him. As soon as he lunged for the impostor, he vanished into the shadows and Yu woke up back in the training hall, with Shinoa and Yoichi leaning over him. Yu had successfully come in contact with a demon without being corrupted by its power.

The chapter ends with Ferid Bathory skipping about in a chapel in Sanguinem, delivering the news of Yu's well-being to a certain hooded vampire. As Ferid draws close, he finds the boy's blade to his throat in a single motion, and cold blue eyes glaring at him--Mikaela Hyakuya is still alive.

Chapter notes

 * The entire school is actually just a laboratory for conducting human experiments to find suitable candidates for Cursed Gears.
 * Yu touches a Cursed Gear without being possessed.
 * Ferid Bathory and Mikaela Hyakuya are still alive.

Characters
List of characters in order of appearance:

Gallery
Manga panels provide a visual overview of the events of Chapter 3 with depictions of the characters, Seraph of the End world and scenarios.