Hell's Handbook - Chapter 13: Elder God

A skilled but unfortunate modern-day assassin was killed in a freak accident. However, he soon found himself summoned to another world by an evil wizard, who had him possess the body of a boy as an experiment. After successfully planning the evil wizard’s downfall, he used the wizard’s body as a springboard to gain influence and power in his new world.

- Description from Novelupdates

When I was reborn in South Korea, I wanted to live my life in peace. Pass the activation system, unlock the 'Big Brother' career, obtain the invincible system, and embark on the road to power

We were a peaceful people living in harmony with nature in the vast lands of Lyréanor, but this peace was short-lived when our people were defeated by the dark elves, an ancient people even older than this world. It is said that they arrived in Lyréanor by boat and since their arrival, chaos has been unleashed. Proud of their obscure power, derived from dark rituals, they have raised gigantic armies of monsters whose name is synonymous with terror: ‘Obscurus’.They went on the rampage, and everywhere they went the grass didn't grow back, life died and disintegrated. Seven, there were seven. The seven dark elves known as the Lords of the Apocalypse. A single gesture from them was enough to strike us all down, a single gesture from one of them was enough to decimate us in blood and terror.And as I, Foster, contemplate the macabre scene before me, the bloodied and deformed bodies of my fellow elfes beings, and as death grips my soul, a strange sign appeared before me.[Ding... Activation of the world tree system]

Rowen transmigrates into the world of One Piece, gaining the Strongest Trait System—capable of plundering the Traits of others!Spotting the golden—shimmering Trait on Koby, Rowen scoffs—You think a clown like you deserves this? Gimme that!From then on—Rowen plunders even more powerful Traits.Marine Headquarters: “He is a monster who devours and takes everything—the most dangerous ability user in history!Engaging him is forbidden without explicit authorization from the highest levels of the World Government!The World's Most Dangerous Man! 'Sky Sovereign' Rowen!DEAD OR ALIVE! Bounty: 3,988,900,000 Beri!”