ゲーム開始前に死ぬモブ悪役皇子に転生した俺　～推しヒロインの妹と幸せになるために最弱魔法【闇刃】を過剰な努力で極め抜いたら、最強のぶっ壊れ性能と化していた件～

Luke is reincarnated as a minor villain prince from a game he played in his previous life—a character destined to die before the main story even begins. In this world, he has an adorable younger sister named Diana, whom he cherishes above all else. However, Luke knows a terrible secret: Diana is fated to fall into darkness and ultimately die. His beloved sister ending up corrupted and dead? There’s no way he’ll allow that scenario to happen! Determined to defy fate, Luke begins training relentlessly from the time he is a baby. By pushing the supposedly weakest magic, “Dark Blade,” to its absolute limits through overwhelming effort, he gradually turns it into a monstrously overpowered ability. This is the story of a mob prince striving to become the strongest in order to protect the sister he loves most.