This manga and the light novels they adapt are a prequel story focusing on Guren Ichinose as a teenager. While content was released after, in-universe, events take place before Owari no Seraph. Both concurrent manga are a part of the Seraph of the End series.
Guren at 14 cuts down Shikigami, their master is a girl. At school the murders of girls is discussed. Shigure and Sayuri check a love hotel. At a crime scene, officer Daigo is joined by Guren. The murder locations would form a pentagram, releasing a curse on Aichi. The reanimated girl attacks and is dealt with by Guren. He knows the caster's location. Daigo used to live with his sister Mizuna and their abusive father. Returning home to her, at night she asks him to embrace her. Recalling her father met Hina Tenkon, he thought sleeping with Mizuna could birth her mother back. Killed, Hina said he could resurrect him, if Mizuna obeyed. He gave her a kitsune mask. Guren arrives, the upper torso of a monster forms out Mizuna's back. A few days later, Daigo thanks Guren for saving Mizuna.
Lying in a field, Mahiru asked Guren if they could marry when they grew up. Parted due to their family, 10 years later, Guren enters First Shibuya High School with Shigure and Sayuri, and has a drink thrown at him, yet they won't reveal their true strength to enemies. Shinya casts an electrical spell tag at Guren. Sitting beside him in class, Shinya knows Guren could have avoided it. He also resents the Hiragis. After Mito asks if Guren was hitting on her, Mahiru addresses the assembly. At his apartment, Guren finds Saitō in the elevator. Guren attacks a potential assassin to see Saito is made up of chains and black fog. He knows the enchantments aren't from the Hiragi family. As Kijima Makoto, from the Hyakuya Sect, Saito asks if Guren wants to destroy the Hiragi with him.
The Hyakuya Sect, since World War 2 was supported by America and now supports Japan. Saito feels his and Guren's goals are the same. Guren won't risk subjection from Saito, who leaves without a fight. Sparring in the school grounds, Goshi judges Guren weak. Mito confronts Guren, then fights Goshi. To ascertain his real strength, Shinya hits Guren who ends up in a medical room. There, Mahiru finds Guren is not talking to her the way he used to. Guren, Shinya with Sayuri and Shigure, then Mito and Goshi meet as a group. Guren is kicked by Seishirō. With him not defending his friends, there are frustrations. Outside a convenience store, Guren sees Saito with Mika. Warning a virus will spread, Saito adds the Hiragi created it. To prevent this, he will go to war in ten days.
On the school field, Shigure fights Mito. Her physical limits enhanced with spells, Mito avoids Shigure’s spell tag kunai until one binds her arm. Freed, Mito knocks Shigure over to win. Mito requests the match be a draw, if it were a real battle the weapons would have been covered in poison. She apologizes to Shigure for her comments before. Goshi's illusions defeat a sword-wielding student. Shinya tells a spell tag user to activate his powers, where it disappears. Guren avoids a spear using student's strikes. Tiring him, Guren finds a parry grants him the victory. Hearing a scream, Sayuri is being beaten by Seishiro. With no one intervening and Seishiro intending to kill Sayuri, his sword is blocked by Mahiru. She does not want to hear apologies if Guren is too weak to protect someone.
With Guren to face Seishiro, he asks Shinya help him withdraw. Noticing a red light, Guren saves Mito as the area is bombed. Hyakuya Sect assassins drop from the sky and begin murdering students. The war has begun two, not ten days later. Saito lands, and Seishiro is saved by Guren. Once Shinya sees his capabilities, with Saito the three have a common enemy of the Hiragis, yet Guren will not bow his head having had enough of that. Assailed, Saito reminds Guren this didn't work before. A spell tag with his Kujakumaru sword delivers a tumorous wound on Saito. On why they targeted students and not the army, Saito divulges the Hiragi family were betrayed. Shinya denies it's him, Saito says it is someone more exceptional than Kureto, so Guren concludes it was Mahiru.
The Imperial Demon’s troops arrive. Guren stabs back into Saito to keep him for information. Mahiru attacks, Guren does not feel the power is human and asks her what is wrong. Her sword creates a destructive black shadow. Mahiru explains it is called cursed gear. Stricken, Mahiru cries she is already possessed by the demon and failed the experiment. Her demonic hand is restrained by Saito’s chain. When Guren attacks Saito, his sword is broken by Mahiru. Not coming with her, the power Guren aims for is different. Mahiru wonders when their paths separated. Revealing the world will be destroyed via the trumpet of the apocalypse, Mahiru then leaves. Mito finds Guren alive, Goshi has survived too. Shinya states Mahiru was kidnapped, a fake reason for why she left school.
A month later after over half the students were killed, Shinya notifies Guren that the Hiragis found out the Hyakuya Sect are the enemy. Student council president Kureto is investigating. Since Guren was seen as weak but survived the attack he is being monitored right now. Shinya and Guren stage a fight so Kureto does not think there is anything between them. Shigure and Sayuri are also aware of Kureto’s background, that he is rumoured to be the next family head. With Mahiru’s betrayal, and the Hyakuya Sect contacting him, Guren thinks right now the Hiragis are losing in this war of information. Making his way to the Hyakuya orphanage, Guren comes across Shinoa who has a message to not come since Mahiru intends to betray the Hyakuya Sect.
At the orphanage, Mika finds Shinoa with Guren. Due to the raid, delayed evaluations are scheduled. In the gymnasium, Guren sees Kureto who questions if he sold information to the Hyakuya Sect. To see if he is hiding power, Kureto orders Shinya to kill Guren. When Mito states Guren has no secret power, Kureto whispers could the Jujo house be traitors as well. Guren fights Kureto at full strength. Encircled by a ring of swords, Guren is struck in the leg. He faints from the anesthesia on it. Bound to a chair, Guren endures torture. Injected, for the torturer to draw back, Guren misdirects that the Hyakuya Sect wanted the Ichinose house to submit. Kureto enters to say Guren’s father came for his son. Tortured, he confessed five hours earlier than Guren until his heart stopped.
Guren will become Kureto’s right hand man or have his whole family killed. Exiting the torture chamber under the gym, Guren isn't talkative to Mito or Goshi. Even if it is Sayuri and Shigure, if Guren cannot abandon them he won’t have the strength to fight the Hiragi or the Hyakuya Sect. Avoiding school, Guren is called by his father. Checking the news, all the animals at Tokyo’s Ueno Zoo were killed. Kureto calls and orders Guren's return to school. There, Guren is held in esteem as Kureto’s subordinate. Seen by Seishiro, yet confiding his secret orders, Guren has him believe that Tenri suspects Kureto a traitor who wants to get rid of Mahiru as a candidate for family head. Seishiro sees it as Tenri ordered Kureto to control the Ichinose house, but really Guren is monitoring Kureto.
The six member group are welcomed by Aoi to Kureto’s office. Kureto states the Ueno Zoo animals dying from poison is a fake story. Imperial Demon intelligence reports it is a Hyakuya Sect testing ground, a crater has been photographed. Kureto judges the war has begun, troops investigating the zoo were wiped out. Rather than pawns, Kureto wants the group to sneak in. Shinya is fine with Guren being team leader as his subordinates only listen to him. Kureto hands Guren the Hakushi demon sword. Near the zoo they change into their uniforms and within find blood but no bodies. Reaching the crater, they find a milky eyed tiger which produces an extendable tongue. Not dying at having its head cut off, instead, spikes grow out of the neck and form into a Horsemen.
The creature attacks, Guren and the squad assail it. Using a blood spell, Guren strikes at the creature, where with assistance he can do this nine times to imbue a curse into it but find the creature can regenerate. Mahiru arrives, and knocks out all except Guren then presses him to kill his friends. Hearing her cursed gear is the only thing that can destroy the creature, Guren touches it to be filled with murderous thoughts. Contacted by a demon for the first time, Guren can slice apart the creature, yet cuts his own own off to deal with the demonic influence. Mahiru is shocked but thinks Guren may be able to reattach his arm using the demon's capabilities. Guren will not go with Mahiru, but bring her with him. Investigating the creature's remains, they sees Ferid and recognize him as a vampire.
A recollection shows Shinya winning a race where he was selected as a candidate to become Mahiru Hiragi's fiance. Taken from his family, Shinya was placed in a school system where those who didn't meet certain criteria such as reaching 30% in exams were disposed of. In matches, students murdered one another. Addressed as Shinya Hiragi, he met Mahiru for the first time. Already having someone she loved, Mahiru suggested she and Shinya pretend to be in love. Where Shinya had not lost to someone before, Mahiru asks how about now. While Shinya is unimpressed with Mahiru's slow speed, he apprehends an illusion. Behind him, Mahiru wins by placing a spell tag on his back. Happy with their arrangement, Shinya wonders who Guren is who Mahiru likes. Outside school he hears Guren is near.
Mahiru attacks Ferid, who strikes her and Guren away before departing with a piece of the chimera. Mahiru is prevented from leaving by Shinya who pretended to faint before. To stop her continuing on her path, Guren and Shinya engage her in combat. Although outmatched, Guren succeeds in cutting off a sample of the chimera claw before she escapes. The squad is conscious yet Guren thinks how he almost killed him. With his arm restored by its power, he is spoken to by the demon. Falling unconscious, Guren awakes from a month long coma. Not returning Sayuri's feelings, he contacts the Imperial Moon Research Institue of Sorcery, and reveals a demon curse thought impossible has been developed, which has affected him. Calling Shinya over the phone, Guren acquires the chimera part from him at school then heads to Nagoya.
Having his blood taken by Mitsuki Iori, and hearing an artificial poison intertwined with a curse is eating away his body, Guren is against human experimentation to find a solution. Injecting herself, Mitsuki finds her left arm swell into a monstrous claw. After Guren severs it before it can spread, the hand develops eyes and can discern that Guren is a demon too. Cutting it apart, Mitsuki and other scientists are fascinated at the power developed. Where they are loyal to house Ichinose, more so since the Hiragis tortured Sakae, Guren remembers Mahiru's words that the world will end at Chistmas. Mid-August now, Guren announces he will decide when to declare war before the end of the year. Sayuri and Shigure are told to ignore what is in the room, and they intend to return to school in Tokyo.
The history between two Hiragi brothers and an Ichinose girl from 500 years ago, where the Order of the Imperial Moon was formed, is discussed. Shinya reveals that the Hyakuya Sect will act tonight, and hands Guren a note. After receiving a call from Kureto to meet him tomorrow, Guren passes a note to Shigure with a location of Hikarigaoka park's tennis court, at 2am. With Shinya, Mito, and Goshi at Guren's house, an apartment fire is reported on TV. A woman calls for Yu, who she wants to kill. They watch as she covers herself in gasoline before attempting to set herself on fire, with Saito seen as well. Thinking that Shinya would have noticed, Guren says they should not act against the Hyakuya Sect due to the danger. Shinya does not appreciate him lying. Sayuri and Shigure eat curry with Guren and don't raise his plans.
After travelling to the park by motorbike, Guren and Shinya make their way to a tree to overlook a meeting location in the distance. Through their night vision goggles they see the Hyakuya agents there come under attack by a vampire where they are executed. After she spots them, Guren and Shinya flee back to the bikes. Losing his after the vampire seizes the wheel, Guren is saved by Shinya and activates a curse to sunder the vampire's arm before they escape. Feeling the vampires can deal with the Hyakuya Sect, Guren wonders if Mahiru is at the centre. Thanking him for saving his life, Guren trusts Shinya a little. Not wanting Sayuri and Shigure near as his blood is cursed, Guren in the bath feels glad he did not endanger their lives as he considers their lives and whether that makes him weak.
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