Futago no Ouji ni Futago de Konyaku Shita kedo "Ja nai Hou" dakara Yami Mahou wo Kiwameru

Also known as 双子の王子に双子で婚約したけど「じゃない方」だから闇魔法を極める / The Twin Princes got Engaged to Twins, but Because I'm "The Other One" I Decided to Master Dark Magic