Chūya Nakahara and Osamu Dazai vs. Arthur Rimbaud

Chūya Nakahara and Osamu Dazai vs. Arthur Rimbaud is a battle fought between Chūya Nakahara and Osamu Dazai against Port Mafia sub-executive Randō during their Arahabaki investigation in pursuit of solving the mystery behind the previous Mafia boss' apparent resurrection.

It happened at an unnamed shipyard building sometime after questioning Randō about Arahabaki, with Dazai and Chūya both uncovering Randō to be the one responsible for the incident.

Prologue
Upon Ōgai Mori's order, Dazai and Chūya investigate about the recent Arahabaki incident and its possible ties to the recent appearances of his deceased Mafia predecessor. The two head out to question Randō, a survivor of a recent Arahabaki explosion in Suribachi City while fighting Sheep members, and recounts the black explosion that killed his subordinates. He vividly describes the experience, illustrating Arahabaki's monstrosity amidst the calmness of the Yokohama sea and how he struggled to survive the explosion using his ability by sheltering himself in his hyperspace.

Later, in an abandoned shipyard building, Dazai and Randō makes some preparations to celebrate Chūya's liberation from Sheep. During their conversation, Randō asks Dazai whether he truly knows the culprit behind Arahabaki, to which Dazai identifies Randō as the one responsible for it. He cites Randō's subtle mention of the Yokohama sea as an inconsistency in his testimony, making him think that Randō described the first Arahabaki explosion before it created Suribachi City's central crater rather than a recent incident.

Chūya suddenly barges in through the window and apprehends Randō as the culprit behind the Arahabaki incident. Since Randō claims having seen Arahabaki himself instead of the previous Mafia boss – which all other testimonies have been stating – Chūya points it out as a lie that makes him identify Randō as the culprit, because he thinks that Randō could not have possibly seen Arahabaki up close. Randō insists having truly seen it before him, and – thinking that Chūya knows where Arahabaki is – asks him where it is, upon which Chūya admits that he is Arahabaki.

Battle
As Chūya asks how and where Randō found him eight years ago, Randō encloses the entire shipyard in his hyperspace and strikes Chūya with a spatial wave. Chūya is unable to defend himself with gravity, as Randō explains that the hyperspace does not adhere to the natural laws of physics. Meanwhile, the previous Mafia boss surfaces and attacks Dazai with his scythe. Randō tells them that the boss is his ability, which allows him to reincarnate one corpse at a time.

Randō then introduces himself as Arthur Rimbaud, a former European Intelligence agent sent in before to capture and kill Arahabaki's host, Chūya. He unleashes multiple explosions with his hyperspace, forcing Chūya to evade all pursuing spatial waves. Chūya eventually grabs onto Dazai as his shield to nullify incoming spatial waves, much to Dazai's chagrin. To counter Dazai's nullification, Randō has the Mafia boss attack Dazai with his scythe, a physical weapon that Dazai cannot nullify.

With Dazai greatly injured and Chūya faced with his natural weakness, Dazai gives up, while Chūya is continuously pursued by Randō's shockwaves. The latter expresses his confidence in killing Chūya to capture Arahabaki once and for all, so Dazai asks to give him some time to persuade Chūya to surrender. In truth, Dazai and Chūya plan their strategy on how to collaboratively defeat their opponent.

Chūya then charges at Randō and evades his shockwaves with rapid speed, eventually landing an attack on Randō. This forces him to defend himself from Chūya's kicks, upon which Dazai appears from behind Chūya to touch Randō and nullify the hyperspace. The previous Mafia boss materializes between them, but, per Dazai's plan, his scythe gets caught in Chūya's rider jacket that is knocked down with gravity, enabling Chūya to smash the old boss.

Dazai heads towards Randō, but the latter instantly collapses the hyperspace and floats all matter inside the atmosphere. Levitating before them, Randō recounts his past mission of taking Arahabaki with him, though he now opts to kill Chūya and make him his ability to peer into his past memories, including what happened to Randō's best friend and fellow spy who infiltrated with him, Paul Verlaine. Unlike his usual method, Chūya decides to bring out his hands and concentrate his ability on both of them.

Amidst Randō's onslaughts, Chūya increases his density to withstand the shockwaves, eventually piercing his fist deep into Randō's abdomen. He quickly lands a series of fists and kicks on Randō's vital areas, plunging him to the ground after the barrage. Randō reveals a thin film of hyperspace hidden in his skin that acts as a buffer against physical attacks, while the old boss swings his scythe at Chūya from behind and pins his left hand to the ground.

Randō lands multiple shockwaves at Chūya, rendering him immobile. As Randō prepares to turn him into his ability, Chūya quickly pierces the scythe into Randō's chest. Dazai holds up a drape that he reveals is tied to Chūya and wrapped around his body underneath, while Dazai holds its other end. With him nullifying Randō's ability, the drape functions as an armor against the hyperspace's shockwaves. Chūya then pulls out the scythe, and Randō falls to the ground. With his vital areas fatally injured and collapsed, the hyperspace enclosing the area begins to disappear.

Aftermath
Randō remembers his best friend Paul, who betrayed and tried to kill him while escaping during their past mission. Randō weakfully bids Chūya to live, regardless of what or who he is and urges him to live as himself, advising that all figures are beautiful in their own right. In his final breath, Randō strangely admits that he does not feel the cold but rather feels warmth in the end. As the hyperspace slowly fades, Dazai and Chūya silently stand before and stare at Randō, who soon succumbs to his death.

Light Novel and Anime Differences
In the anime, Dazai and Chūya defeat Randō through a different collaborative approach. As they notice Randō controlling the hyperspace isolating Dazai with the old boss to keep him from Chūya, the latter decides to use both his hands and concentrate his ability on both of them. They soon reach out to each other and clasp their hands, with Chūya bracing himself against Randō's hyperspace and Dazai nullifying its boundary.

Moreover, in the anime, the battle between Dazai and the old boss is expanded. Before reaching out to Chūya, Dazai holds his own against the old boss and shields himself from his scythe with a metal encasement hidden beneath his bandages. As the entire hyperspace is soon nullified, Chūya knocks out Randō with a single punch. This causes the old boss to dissipate as well shortly after Dazai punches him with his metal-encased arm, retorting it as his way of showing his thanks before the old boss disappears for good.