Demons' Crest is a Manga/Manhwa/Manhua in english language, ACTION series, english chapters have been translated and you can read them here. You are reading chapters on www.mgeko.cc fastest updating comic site. The Summary is

In the year 2031, in which elementary school student Yuma Ashihara participates in a beta test of the world's first full-dive VRMMO-RPG called "Actual Magic (AM)", only to lose consciousness while playing and wake up in a world where the game and reality have merged. A death game then unfolds in "mixed reality."