Kikan Shita S-Kyuu Boukensha no Kuesuto Seikatsu

Original Webtoon

Dungeons began to suddenly appear throughout the world
Kyomi Aki became an S-Class adventurer assassin with his tremendous speed.
Unfortunately, assassins were considered as a failure class.
His class was becoming harder and harder to master the more he advanced.
The class itself had a low attack power and mana, so he needed to always rely on someone else.
One day, Aki encountered a dangerous monster that could wipe out the entirety of humanity.
He somewhat managed to subjugate the monster, but lost his life in the process.
Suddenly, he awoke and found himself having regressed 20 years into the past.
He also received an additional “Bonus Quest” as a reward for solo killing the monster.
With his knowledge from his previous lifetime and this “quest,” Aki resolved himself to redo everything to become the strongest adventurer.