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～