High School DxD - Volume 21 Chapter 6.1

Violet skies, a world undergoing massive changes, a deficient moon, interlinking voids…
When the world experienced a cataclysmic event,the majority of living things ceased to exist. Although humanity survived by hiding in the last safe zones, humans had to face mutated dire beasts and colossal source beasts that could break through the space barrier and enter the earth at any second.
Luo Yunyang, a young man who grew up in those calamitous times, obtained an attribute regulator and set out to walk the path of supremacy!

- Description from novelupdates

After 140 years that gates and dungeons appeared on Earth and a system allowed humans to use special abilities, Matthew an 18 years old who just got out of high school will seek adventure inside those perilous places. However, instead of being able to select his own class as he progress the levels, destiny has another plan for him.

The worst thing that Shui Ruoshan think he ever did? He wrote the story of “The Strongest King in History”.

Have you ever thought about how your life would turn out at the age of 18?Well before I could even say that I was an adult to even say I was going to conquer it.The world had seemed to already be determined of what mine and the rest of humanity would be when the first Leviathans was made public to our world.