Not long after Ougai Mori takes the reins of the Port Mafia, he orders a young Osamu Dazai to investigate an ominous rumor of something called “Arahabaki.” Whoever or whatever it is seems to have some connection to Mori's deceased predecessor—and the new boss would very much like for the dead to stay buried. Along the way, Dazai runs into a gravity manipulator named Chuuya Nakahara, leader of the Sheep gang of youths. Together, the two will one day achieve infamy as the fearsome duo known as Twin Dark, but for now, they'll have to learn to put aside their differences if they want to get to the bottom of this mystery…
 
(Source: Yen Press)
 
Note: The novel was later adapted into the Bungo Stray Dogs anime, consisting of episodes 26 to 28.

The third season of Bungou Stray Dogs. 
 
The White Tiger and the Black Beast – Nakajima Atsushi and Akutagawa Ryunosuke’s fight against Francis F. brings an end to the great war against the Guild. Life goes on as normal in Yokohama, thanks to the continued truce between the Armed Detective Agency and the Port Mafia, who, together, saved the city from ruin. But there are still rumors of Guild stragglers and other crime organizations making their way in from abroad. Meanwhile, Dazai Osamu had premonitions of another impending disaster. Lurking in the darkness is the Fyodor D., leader of pirate organization Rats in the House of the Dead, his dreadful plans on the verge of execution.
 
(Source: Crunchyroll)

Having been kicked out of the orphanage, a despairing young man by the name of Atsushi Nakajima rescues a strange man from a suicide attempt--Osamu Dazai. Turns out that Dazai is part of an armed-detective agency staffed by individuals whose supernatural powers take on a literary bent! 

(Source: Yen Press)

Not long after Ougai Mori takes the reins of the Port Mafia, his accomplice Osamu Dazai has a dreadful run in with Chuuya Nakahara, a boy known as the Sheep King who is capable of manipulating gravity. But when strange rumors about the mysterious being Arahabaki start spreading throughout Yokohoma, the duo must put aside their differences and get to the bottom of things. Before they achieved infamy as the fearsome pairing Twin Dark, Dazai and Chuuya were just boys. Was their first encounter a harbinger of hope, or an ill omen of things to come...?
 
(Source: Yen Press)

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