Regressor of the Fallen Family - Chapter 171

Instead of struggling meaninglessly, he acknowledged his place.

Chi Lang transmigrated to the Middle Ages as a third-rate magician in a third-rate magic academy. His magical aptitude is zero. But he has a good roommate.

His roommate is good-looking.

His roommate is good-natured.

His roommate helped him escape from danger several times.

His roommate never complains.

Chi Lang felt like he liked his roommate a little bit, and then his roommate helped him solve his ‘single’ problem.

It wasn’t until much later that he realized that his roommate was a remarkable figure.

——He was one of the demons sealed by the magician known as the “God of Magic”… And Chi Lang became the one who unlocked the seal of the Demon Lord.

- Description from Novelupdates

The world was once at war with 10 alternate worlds. This was known as the Concept War and our world won 60 years ago. All of this was concealed so no normal people are aware of this fact in the current time.

After the death of his grandfather, the high-school student Sayama Mikoto is suddenly summoned by the giant corporation IAI. He learns this world is headed for destruction due to the acceleration of the Minus Concepts. To prevent this, negotiations must be undertaken with the survivors of the other parallel worlds so their 10 concepts can be released.

And so Sayama is caught up in the final battle to clean up after the Concept War which left behind so much enmity.