Death March kara Hajimaru Isekai Kyusoukyoku (WN) - Book 1 - 1-2. Written Dream Read as Ephemeral

Lin Chen was persecuted by his childhood sweetheart and accidentally obtained an ancient legacy. From then on, he embarked on a path against the heavens!“I am a sovereign! Those who insult me will be killed, those who obstruct me will die!”

Time flies quickly; may you enjoy a cup of drink.I know not the height of the green mountain, nor the depth of the earth.Only watching as the seasons change, that it took people's lives.

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