I'm at Level 30, But I Can Slay Level-9999 Beasts in One Hit! - Chapter 29: Slaying the Goblin Giant

This is a story of an ordinary boy gaining unusual powers from his nightmares… It all started when Lin Sheng experienced something strange on what he thought was just another normal day. He begins to have recurring dreams that lead him on an incredible journey of discovery as he encounters spine-chilling monsters and elusive secrets.

- Description from Novelupdates

Lin Jiang travels through the world of cultivation and obtains the Lifespan Check-in System.Check-in for a day, gain one year of lifespan.This nearly infinite lifespan allows him to live tens of thousands years, bearing witness to numerous lives passing by, their joy, grief, and everything in between.He has persisted from the Stone Age to the Cosmic Age and ranges from cultivation to science. He is there, always there, as the eternal guardian of mankind.(Not a dark and thrilling story, be cautious)

Every human on Earth was transformed into a summoned monster and was summoned to another world.

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.