Game Kaishi Mae ni Shini Mob Akuyaku Ouji ni Tensei shita Ore

I Reincarnated as a Mob Villain Prince Who Dies Before the Game Even Starts ~In order to be happy with my favorite heroine's sister • I excessively mastered the weakest magic • [Dark Blade] • and it ended up becoming the strongest • most broken magic • 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