Tensei Shite Inaka de Slowlife wo Okuritai - Chapter 30 Fruit Water Affects

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

When a person dies, they either die light as a feather (aka unimportant) or heavy as Mt. Tai (aka very important).

Duwei is born in this new world from Earth without making any sound, he does not cry, he just stares at the people around him. As he grows he does not speak and curses this new world in which he is forced to live.

His father a great general of insurance he must have aptitude for something, nevertheless he is not more than trash. Ignored by his family as the “ret*rded” heir.

This is the story of your journey to discover the truths of this world and use the knowledge of your past life to remove your title of ret*rd, even if you have to become a devil.

- Description from Novelupdates