Read manhwa The Dark Mage’s Return to Enlistment

Minjun Kim, an ordinary high school senior in Korea, was suddenly summoned to another world, where he became a dark mage. Determined to return home, he overcame all sorts of hardships and saved the other world with black magic before leaving all his wealth and glory as a hero behind to return to Earth. However, before he could fully enjoy his life, a problem arose. A dungeon break had occured and monsters came pouring out, threatening his comfortable days. Therefore, Minjun, who had just returned to earth, had no choice but to… enlist on the very next day!!!
https://teenmanhua.com/manga/the-dark-mage-s-return-to-enlistment