Game Kaishi Mae ni Shinu Mob Akuyaku Ouji ni Tensei shita Ore: Oshi Heroine no Imouto to Shiawase ni Naru Tame ni Saijaku Mahou "Dark Edge" wo Kajou na Doryoku de Kiwamenuitara, Saikyou no Bukkoware Seinou to Kashiteita Ken

Luke has reincarnated as a mob prince from a game he played in his past life. In this world, there is Diana, the cutest little sister in the world. As it turns out, Diana is fated to die after falling into darkness. His beloved little sister will die at the end of a dark path? Luke is determined to smash that scenario to pieces. He has been striving ever since he was a baby to defy that fate. This is the story of a mob prince aiming to become the strongest to protect his beloved little sister. (Source: MangaPoke, translated)