Bungo Stray Dogs: Storm Bringer

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)

The story is set after the "Arahabaki Incident," and one year after Chuuya Nakahara joined the Port Mafia. A detective from Europe named Adam appears in front of Chuuya, with the aim of protecting Chuuya from Paul Verlaine, also known as the king of assassins. Paul claims to be Chuuya's old brother, and he vows to murder everyone Chuuya cares about. Based on LN Vol 8.