One Piece: King of the World - Chapter 3 - : King Engine?

220 years ago, in the legendary Dragon Demon War, Hero Azell ended the war by killing the evil Dragon Demon race’s King Atein. He was able to save the population, but as a consequence, he was cursed. While he was dying, the high magician suggested a gamble that might save his life.

The oldest myth between heaven and earth reappears in the world, countless spaces begin to slowly interweave, and the dark side of the world quietly emerges.

Advocate the friars who are weak in flesh and blood and eternal in spirit. Showing a tyrannical body, there is my invincible martial artist.

Looking at the changes in the world, Lu Ren exhaled in a panic.

“Ding, because of your uninterrupted breathing for 26 years, you suddenly realized something, and successfully realized the breathing technique.”

Hearing the prompt sound in his mind, Lu Ren waved his sword hesitantly.

“Ding, because you were blindly poking a few times with your sword, you suddenly realized something and successfully learned basic swordsmanship.”

After a long time, he looked at the army invading from another world, raised his long sword and slashed it out.

In an instant, the endless air waves rolled back, and the mountains and rivers where they passed collapsed, and the rivers fell.

Then the army was annihilated and bombarded towards the different world. The entire other world was completely shattered by the explosion of infinite fire, and slowly disappeared into the air.

- Description from novelbuddy

To correct the mistakes of his past, a young man is forced to narrow the space between science and witchcraft, as he ventures on a quest to save his world from complete destruction.

A world filled with heroes with superpowers. A world attacked by calamity fiends. A modern world filled with wonders and dangers.

Shi Xiaobai, a child from normal Earth, walked into such a world, proclaiming to be its king. But at the first signs of danger, he is forced to make a choice, one which he cannot refuse for time would repeat, making him face the choice again.

He is not humble nor is he modest, but neither is he delusional. Yet, the Absolute Choice seems to make fun of him. Will the joke be on him or will he laugh at his opponents? There will be many watching his escapades, and there will be some joining him in his adventures.

To see through the world and to grasp its truth, that is the destiny of the King.

- Description from novelupdates