Bungou Stray Dogs: Stormbringer

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.