Mahou Shoujo Karen -Ingoku no Coliseum ni Kieta Akuochi Shimai-

Karen, the protagonist, is a magical girl fighting for justice. Together with her younger sister Ao, she had been dedicated to defeating monsters. One day, they were suddenly ambushed by the Demon Lord, and her sister went missing. As Karen was consumed by despair, a mysterious messenger appeared, claiming to know the whereabouts of her sister. The messenger invited Karen to a coliseum overrun by monsters. [Translated from Website]