I regressed and became the Sword Ice King - Chapter 275 - - Festival 6- Class Two Gold

The worst student of Westroad Academy. The disgrace of the Blake House. A new soul enters the body of Frey Blake, who couldn’t overcome his miserable life and chose death instead. “What’s with this body? I’ll have to work on tuning it from scratch.” The Great Mage Lucas Traumen, with the body of Frey Blake, gains a chance at revenge!

Eight hundred years ago, Xie Lian was the Crown Prince of the Xian Le kingdom. He was beloved by his citizens and was considered the darling of the world. He ascended to the Heavens at a young age; however, due to unfortunate circumstances, was quickly banished back to the mortal realm. Years later, he ascends again–only to be banished again a few minutes after his ascension.

Now, eight hundred years later, Xie Lian ascends to the Heavens for the third time as the laughing stock among all three realms. On his first task as a god thrice ascended, he meets a mysterious demon who rules the ghosts and terrifies the Heavens, yet, unbeknownst to Xie Lian, this demon king has been paying attention to him for a very, very long time.

- Description from Novelupdates

A 34-year-old NEET otaku, chased out from his house by his family, found that his life is reaching a dead end. He then recalled that his life could actually have been much better if he had made better choices in the past.

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.