One Piece: Unreal Adventure - Chapter 23: White Slash!

Yggdrasil, the World Tree of Norse Mythology, suddenly appeared on Earth. And with it came demonic creatures that ravaged entire cities.Although not all hope is lost, due to the System that a few special humans had gained.In this new world where only the strong survived, Min JaeHyun scrapes by regretting the wrong choices he’d made in the past… when one day, he gains the only Mythic item in the world.How will this item help him? And can he change his wretched fate?

After thousands of years of silence, the world of cultivation once again welcomed the golden age.

As a promise, he went out into the world as an assassin. 3 years later, after becoming the Supreme Assassin, he gave up on everything he had and returned to the ranks of ordinary people. But one day, Kismet, a mystical game created by the National Research and Development had appeared.

Enigmatic hands, spectral frame, and one cunning strike just like the swing of the Grim Reaper. The Supreme Assassin takes on the mystical world, and how his legendary tales sets off, progressively ascending the altar to becoming the best.

- Description from Novelupdates