Story
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.
Synopsis via AniList (English).
Preview
From the anime adaptation: Bungo Stray Dogs 3.