Start in DxD as Motohama: Leveling Up in Another World - Chapter 36: A level (I) beatdown.

Yun Bixue is in dire straits—

Living life in this new world was already hard enough....So why?Why is the villainess of this world obsessed with me?

The inexplicable existence expands the earth and stars a hundred times, and the whole world is digitized by the game. The ancient existence began to return, and there were alien races in the chaos.

This is the struggle for hegemony between the fierce soldiers of the Great Qin Dynasty and the Yulin Army of the prosperous Tang Dynasty, and also the collision between the military soldiers of Wei Dynasty and the trapped camp.

When the Spartan Warriors met the Yellow Turban Warriors, when the Holy See’s Crusades met the Mongolian Iron Cavalry… Elephant Soldiers, Iron Buddhas, Viking Pirates… Humans, Beasts, Alien Invasions… …Who is the strongest civilization in the heavens?

- Description from novelbuddy

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.