Calgrande is a magical academy that the nobles and elite magicians attend.

A world of magical fundamentalism, where the strongest talents for the national military gather. Yuuma, a genius magician who once before saved another world, died and reincarnated to this world, and is now attending this academy for the sake of achieving a certain goal. Yuuma, who is considered the hero candidate of this generation, was living happy days with his genius sister-in-law who is also a brocon, as well as with his other sister-in-law who is a spoiled angel. However, all of this changed during a extracurricular trip to the demon world. When the evil parts of this world bare their fangs against his precious sister, the magician from the otherworld is unleashing his true power!

(Source: Kadokawa, translated)