21 century retrogression 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

Ha pung gae, old Taoist monk, who has lived since the end of joseon Dynasty, unexpectedly succeeds in the counter-retrogression movement. However, even if successful, he has only about ten years left in his remaining lifespan. In order to avoid death, he must become immortal!