One-shot by KaguraBachi's mangaka, unrelated to main series.
In a dimly lit setting beneath a railway bridge, Roku meets Lucille, a mysterious entity from hell. Lucille claims superiority over humans, showcases mind-reading skills, and proposes a pact: Roku can avoid death for a century and gain its powers by seeking vengeance on worthy targets. Roku reflects on his past as an assassin with his lifeless friend Machi. After a violent clash with his father and fellow assassins, Lucille helps Roku decide, and he ultimately eliminates his enemies with newfound determination.