Arahabaki Incident

The Arahabaki Incident (事件,) or the Impostor Predecessor Incident (偽り者先代首領事件,) refers to the incident of the Port Mafia's deceased predecessor appearing in Suribachi City near Yokohama, a plot made by Arthur Rimbaud to lure out the true Arahabaki, Chūya Nakahara.

The root of the incident happened eight years ago at the end of the war, where a former national defense force was secretly researching an artificial ability-derived life-form known as Arahabaki.

Background
During the research of Arahabaki, Rimbaud and Paul Verlaine, two European agents, plotted to steal Arahabaki in a Japanese military facility as ordered by France and managed to successfully steal it, specifically Chūya who served as Arahabaki's host. However, sympathizing for the boy as an ability-derived life-form himself, Verlaine betrayed and shot Rimbaud from behind, though to little avail. This led to a vicious fight between the two, which caused a light from their battle to scorch the night sky and was eventually followed by an explosion in the area.

During the battle, Rimbaud was forced to kill Verlaine, and the large battle caught the attention of military's tracking unit. During Rimbaud's moments of being surrounded by the military, he attempted to absorb Arahabaki, which backfired on him and ended up breaking its seal.

The breaking of Arahabaki's seal led to it unleashing its true powers that surpassed human comprehension. Arahabaki showed its true form, covered in black flames, and destroyed everything in the area: the military, the research facility where it was contained, and the surrounding land which created Suribachi City.

Having failed to absorb the true Arahabaki, Rimbaud spent the following years to slowly regain his memories and abilities. As such, he planned to trap and lure out the true Arahabaki.

Summary
Shortly after Ōgai Mori assumes the role of Mafia boss after killing his predecessor, rumors of the latter's reappearance in Suribachi City have spread. He then sends Osamu Dazai to investigate the rumors. Accompanied by Ryūrō Hirotsu, Dazai finds three eyewitnesses confirming the rumors. He is later accosted by Chūya and demands to tell him what they know about Arahabaki. Soon after, the previous boss appears in an explosion of black flames, marking it as his fourth alleged reappearance.

At the Mafia headquarters, Mori soon fills in Dazai and Chūya about the previous boss' earlier emergence in the Mafia's nearly-impregnable vault. The previous boss declared having been summoned back from hell by the beast Arahabaki and threatened to enact his revenge on Mori. The latter then sends in the two to continue the investigation, leading them to seek out Mafia sub-executive Randō, who was said to have survived an explosion by the previous boss a week prior.

Dazai and Chūya find Randō's mansion in black flames due to an explosion caused by Gelhart Security Service forces. After Chūya repels them, Dazai surmises that the black explosion has been staged and is likely to fuel rumors of the previous boss attacking Randō, a supporter of Mori. After listening to Randō's account about surviving the attack of Arahabaki itself rather than the previous boss a week ago, Dazai later names Randō as the perpetrator who started the rumors, noticing that Randō's testimony was about the very first Arahabaki sighting before Suribachi City was even created and therefore knows about it. Chūya also eventually pinpoints Randō as the culprit and reveals himself to be Arahabaki.

Randō, introducing himself as Arthur Rimbaud, summons the previous boss using his ability and battles Chūya and Dazai, respectively, in an attempt to kill Chūya. In the end, however, the two collaborate into defeating Rimbaud, leading to his demise and thereby purging the rumors of the previous boss' reappearance for good.

Aftermath
After his betrayal to the Mafia, Randō is branded as the solo perpetrator behind the entire incident. As common Mafia procedure, his house was burned down, and his possessions were thrown into the ocean. His body was also left exposed to the elements for a week before being buried in a rural public cemetery.

The details of the Arahabaki Incident are later documented by Ango Sakaguchi in an exclusive report. Furthermore, the incident catches the attention of Verlaine following Rimbaud's death and comes to Japan seeking Chūya.