MASIN

A hundred children were trapped on Hengshan. Only one walked out alive—a boy named Dan Hyeongwoo. He survived the hell created by the Cheon Gija and gained power worthy of being called that of a demon god in return. But the world felt unfamiliar to him, and its rules made no sense... So, he becomes an escort warrior of the Henan Escort Agency, relearning what people are and what the world is from the ground up. Everything he does is for one single purpose: the day he will take revenge on Cheon Gija.(Source: Naver, translated) — Description from AniList community (anilist.co).