Bungo Stray Dogs, Vol. 6 (light novel)

What if the Port Mafias attack dog, Ryuunosuke Akutagawa, had joined the Armed Detective Agency? Likewise, what if Atsushi Nakajima belonged to the Port Mafia? In the parallel universe of Bungo Stray Dogs Beast, Akutagawas desperate mission to rescue his younger sister from the Port Mafias clutches brings him face-to-face with the organizations fearsome White Reaperaka Atsushi Nakajima. What does fate have in store for this alternate tale of darkness versus light?