The Regressed S-Class Adventurer’s Quest Life

A brief description of the manga The Regressed S-Class Adventurer’s Quest Life:

Dungeons appeared out of nowhere all across the world, and because to his exceptional speed, Kyomi Aki evolved into an S-Class adventurer assassin.
Assassins were regrettably viewed as a failing class.

The more he improved, the tougher it was to master his class.
Because the class’s attack power and mana were so low, he was always dependent on other people.
Aki came onto a terrifying beast one day that had the potential to exterminate all of humanity.
He died trying to defeat the monster.
All of a sudden, he woke up and realized he had traveled back in time twenty years.
In addition, he was rewarded with another “Bonus Quest” for taking out the monster by himself.

With this “quest” and his knowledge from his past life, Aki decided to start over and become the greatest explorer.