Black Gate Kanin no Gakuen

Narifumi Hosokawa prepares for his high schools culture festival with the Kadomori twins, Shizuku and Kasumi. While everyone is distracted, Shizuku decides to open a mysterious gate through the use of magic and travel to an alternate world. She has broken the oath of her familys heritage, as to protect the Black Gate from those who seek the enormous power that lies within the world.