Arthur Rimbaud/Synopsis

Fifteen Arc
Port Mafia sub-executive Randō uses his subspace ability to restrain Chūya Nakahara. Ōgai Mori requests he leave, to which Randō is wary of freeing Chūya so easily. In response, Mori questions if he's cold, and Rando admits he is freezing, unable to deal with the cold whatsoever, and Mori convinces him to free Chūya and leave.

During their investigation of Arahabaki, Chūya and Osamu Dazai arrive at a dilapidated Western-style building where Randō lives. Outside, they defeat Gelhart Security Service members, while Randō inside shivers in front of a fire, trying to stay warm against the cold wind blowing in. Randō and Dazai feed the fire with old, broken pillar material and even an expensive art piece. Dazai elaborates that GSS wanted to kill Randō to spark internal conflict between Mori's faction and the former Mafia boss' supporters, as Randō's loyalty to Mori would raise suspicions inside the Mafia. Randō also notes that GSS has deep ties with the North American Guild.

Chūya and Dazai request Randō to testify his eyewitness encounter of Arahabaki one week ago in Suribachi City. Randō explains the Port Mafia and the Sheep fought once again when Arahabaki suddenly manifested and decimated the area. Despite it vaguely resembling the former boss, Randō describes Arahabaki as a quadrupedal, black beast with flaming fur, a tail, and hellish eyes. Although its figure resembled a human on all fours, something about it was clearly inhuman, convincing him that it has no distinct, intentional will for all the chaos and age it embodies and simply existed.

Although he attempts to rationalize the phenomenon as an enemy's ability, he asserts that it could not have been created by a human due to its intense, tremulous heat. He also recalls Arahabaki's fiery existence causing massive heatwaves that distorted his surroundings, with only Yokohama's ocean untouched. Before passing out at the sight, Randō claims that Arahabaki's mere existence brought about destruction.

Soon after the Sheep dissolve their ties with Chūya, Randō and Dazai prepare a "party" in an abandoned building. After a notably long ribbon catches Dazai's attention, Randō comments that the set-up looks more like a site prepared for murder than a party. Dazai explains that it is to celebrate Chūya's "freedom" from the Sheep and proceeds to explain his elaborate ideas for celebration. Dazai then asserts that it would be best for the Sheep and Chūya to reconcile, given the flaws of their current structure. Randō then learns about the "undercooked meat theory" that Mori taught Dazai. In a nutshell, the theory relates to all opposing parties reaching the same optimum solution at the same time, rendering its effectiveness null and irreversible. To Dazai, this explains the Sheep's situation perfectly.

Rimbaud agrees and muses at the thought of everyone making a rule to not "eat undercooked meat" (stop fighting and rid their weapons), as their violence is unnecessary. However, Randō is not so naïve that he finds this plausible, as there is always someone that benefits from violence, forcing others to retaliate with violence. Randō views this as the natural way of the underground world. Dazai notes Randō knows a great deal more than him, and Randō points out he only rose to such a high rank thanks to Mori, whom he feels only gratitude towards. Finally, he asks Dazai who the culprit is.

Dazai easily points the finger at Randō himself, much to the man's shock. He continues to mock how any accused person would react, to which Randō cannot interject, instead of asking how Dazai will refute such protests. Their back and forth goes on until Randō urges Dazai to elaborate on why he accuses him to be behind the Arahabaki sightings, and Dazai points out a fatal flaw in Randō's account of Arahabaki by mentioned the ocean. As a valley formed from a massive, spherical explosion in the ground, Suribachi City has no visible view of the ocean and therefore has led Dazai to believe Randō truly did see the ocean not a week ago but eight years ago during Arahabaki's very first sighting and the subsequent creation of Suribachi City, when he could have truly sighted the ocean. Having been found out, Randō admits he is behind the sightings of Arahabaki and the former boss's "revival".

Chūya instantly erupts and subdues him, having discovered Randō to be the culprit not because of the ocean contradiction, but because all other witness accounts saw the predecessor's visage. Meanwhile, Randō describes Arahabaki itself, which Chūya considers impossible. Randō asks if it is because he does not believe in gods, but Chūya asserts gods do exist and that Randō should not have been able to see one. Hearing someone say they know Arahabaki exists unsettles Randō, and Chūya surmises that Randō may have seen it eight years ago, given his accurate recalling of it.

Randō urges Chūya to tell him where Arahabaki is, mentioning that its sheer destructive power, its existence beyond human comprehension and wisdom, and the entity that emitted such heat it burned him, is what Rimbaud is after. After his pleas, Chūya reveals he himself is Arahabaki. The revelation does not shock Randō and in fact admits he has suspected it from the start. Chūya elaborates on how he came to be, gaining a personality and will of his own, and recalls a mysterious hand "pulling" him from the bluish-black darkness Arahabaki once knew alone. He pursued the case to find out where he was found, why he was pulled out of the darkness, and how Randō saw Arahabaki's true form outside its vessel.

Randō amusingly agrees to answer Chūya's many questions. He summons his subspace so large it captures Chūya and, without coming into contact with him, Dazai. Randō remembers his true name, Arthur Rimbaud, and his intent to kill Chūya recently, and as his ability exists outside natural laws, Chūya's ability is rendered useless. Identical to the rumors spreading through Yokohama, the predecessor's likeness appears armed with a scythe, revealing Rimbaud's ability, Illuminations, is able to absorb corpses into his subspace and control it as an ability.

Rimbaud explains he was a European spy sent to investigate a high-energy being in Japan as part of his mission. Rimbaud reintroduces himself and reveals his mission to kill Chūya and have his corpse replace the former boss as his ability.

With the predecessor wielding a physical weapon, No Longer Human is ineffective to nullify the predecessor's attacks, injuring him. Rimbaud laments having to kill Dazai, but he cannot afford to leave Dazai alive to inform the Mafia of his betrayal. The battle rages on, but even Chūya is overpowered by Illuminations. Rimbaud expresses no pleasure in attempting to kill both of them and admits that he made a mistake eight years ago, ending up surrounded by the enemy.

Rimbaud recounts that the ability-derived life-form he was using at the time could not defeat his enemies, and he devised a plan to absorb Arahabaki from Chūya's body. However, he claims to have pulled out Arahabaki's safeguard, Chūya, though he actually absorbed its seal. This resulted in the calamitous being's complete form decimating everything and creating Suribachi City. Dazai negotiates five minutes to convince Chūya to give up, but Rimbaud grants them only two. After the boys' planning, the fight continues. Despite overpowering the predecessor, however, they're cornered by Rimbaud's subspace once more.

Floating in the air, Rimbaud mentions his partner, Paul Verlaine, whom he worked with in attempting to obtain Arahabaki, but his amnesia allows him only to mention that something terrible happened that led to them being cornered, forcing Rimbaud to try using Arahabaki's power for himself. He admits he has no idea what Chūya nor Arahabaki are, as the explosion destroyed any remaining knowledge of the facility's experiments. His goal is to kill and absorb Chūya's corpse to recall those memories and understand everything, along with remembering what exactly happened to him and his lost friend.

Upon hearing Rimbaud's objectives for the sake of his friend, Chūya finds himself viewing Rimbaud in a more positive, respectful light, finding no fault in Rimbaud doing anything for his comrade. This newfound knowledge allows Chūya to concentrate his ability and overcome the subspace, slowly making his way to Rimbaud before attacking him. However, Rimbaud is unharmed, retaliating with his ability.

Just then, the boys reveal their trump card in the form of the long garland from earlier: Chūya had hidden it under his clothes with Dazai holding the other end, nullifying Illuminations' effect on Chūya. When Illuminations attacks, Chūya impales Rimbaud with the former boss' scythe.

The pain and shock cause Rimbaud to regain his memories from eight years ago. Rimbaud recalls Verlaine betraying and trying to kill him from behind. The two fought a fierce battle, and Rimbaud, believing himself as the victor, claims to have killed his own friend. The battle had alerted the military, leading to Rimbaud's desperate attempt to absorb Arahabaki.

Covered in blood, Rimbaud requests Chūya to live with his own will even if he is but a mere pattern. After all, he believes that all human beings and all of life are beautiful patterns upon the material world. Dying, Rimbaud smiles and notes how strange it is not to feel cold. His last breath recalls Verlaine, wondering if his former partner felt the same warmth upon death.

King of Assassins Incident
Following his death, Rimbaud apparently uses his ability on himself, turning him into an ability-derived life-form. Maintaining his skills and memories, Rimbaud stays in Japan waiting for Verlaine.

One year later, after Demonic Beast Guivre's rampage that claimed Verlaine's life, Rimbaud finds him encloses him in his subspace, then uses his ability on himself ad infinitum to generate a self-contradicting singularity. Once successful, Rimbaud revives Verlaine by giving the singularity to him in place of Guivre, a feat that Rimbaud describes to be his gift for Verlaine as he apologizes for pretending to understand him in their lifetime.

Shortly after, Rimbaud greets him a happy birthday, glad that Verlaine had been born and that he had the honor of meeting him. The subspace condenses until it is sucked into Verlaine's heart, soon vanishing along with Rimbaud himself.

Legacy
After Rimbaud's death, his home is burned down, his belongings thrown to sea, and his body left to the elements for a week before receiving its burial in a rural cemetery. His actions are also disclosed to be betraying the Mafia and the Arahabaki Incident pinned solely on him.

Rimbaud leaves behind a profound legacy and influence over Chūya. One month after Rimbaud's death, Chūya visits and sits atop Rimbaud's wearing grave, calling him troublesome even in death, as the Mafia has discarded all his investigative records of Arahabaki. Chūya nevertheless admits that Rimbaud would have never told anyone about his findings had he survived anyway and instead promises to visit his grave soon. Later on, as a custom for joining the Mafia, Chūya is given an item that belonged to the one who recruited him to the organization. Hence, Mori gives Chūya with a hat that belonged to Rimbaud.

Rimbaud's passing and posthumous act for Verlaine made the latter largely uninterested in the outside world following the King of Assassins Incident. As such, Verlaine lets himself be locked up in the Mafia basement, where he trains young assassins and claims to await "a storm".