Harry Potter: A Certain Ancient Rune Professor of Hogwarts (TL) - Chapter 716

Lambert, one of the great generals from Regios kingdom was suspected for planning a rebellion against his lord at the end of unification war, he died an unnatural death. Two hundreds years later, when the era has changed and Regios kingdom entered it’s peaceful era, he accidentally revived as a demon called Undead Knight.. Lambert whose entire body was wrapped in a flashy Demon Metal[Organ] set out on a journey to punish the wrongdoers while enjoying the peaceful sight of Regios kingdom. But…….. This peaceful era was basically an easy mode for Lambert, the greatest general of the chaotic era.

The world experienced a cataclysmic change, and countless spatial gateways appeared out of thin air.

Exotic beasts of all shapes and sizes swarmed out of those aforementioned spatial gateways, forcing the humans to face the crucial moment for their survival.

In order to search for a chance at survival, brave warriors from the army and scientific researchers risked their lives and entered the world beyond the spatial gateways. To their surprise, they discovered that there was a chance of obtaining beast cards by killing these exotic beasts in their world.

Those with extraordinary spiritual strength are able to utilize the beast cards, perhaps transforming the cards into the beasts’ combat bodies while they were still alive, or transforming them into weapons or defensive gears, or…

A new occupation came into existence following the changes, known as beastmasters.

A transcender with two souls fused into one found himself in this magnificent new world full of waves, rising step by step as he ascends into the seat of a godly beastmaster.

- Description from Novelupdates

If I were to fall into a game, I wish I would fall when I was young.

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.