The Dark Mage's Return to Enlistment

Min-Jun 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 occurred and monsters came pouring out, threatening his comfortable days. Therefore, Min-Jun, who had just returned to earth, had no choice but to… enlist on the very next day!!!

(Source: Naver, translated)