From Yen Press:
Seven years before the events of Bungo Stray Dogs, a young Osamu Dazai is ordered by Ougai Mori to investigate an ominous rumor there have been sightings of Mori’s deceased predecessor, and the new Mafia 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 teenage Sheep gang. Together, the two will one day achieve infamy as the fearsome duo Twin Dark, but for now, they’ll have to learn to cooperate if they want to get to the bottom of this mystery.