Black Gate(Black Gate: Kanin no Gakuen)

Narifumi Hosokawa prepares for his high school's 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 family's heritage, as to protect the Black Gate from those who seek the enormous power that lies within the world.