Immortal Weakling is a Manga/Manhwa/Manhua in (English/Raw) language, Action series, english chapters have been translated and you can read them here. You are reading chapters on https://mangabtt.com fastest updating comic site. The Summary is
After retiring as a vigilante so he can live a peaceful life, Yagura gets trapped in a survival game that is far more than what it seems. Now he must come out of retirement and train a new group of heroes to protect his city — but will it really pay to be a hero?