I really need some help

I need help finding a manga? Weboon? (I don't remember) about a guy (mc) who has to other siblings both men but one is missing (the oldest one?) Then the mc decides to quit his job and join his other brother that is not missing to find the missing one, and to find the missing brother they use a video game because that's how their older brother went missing? And in the game the MC has a skeleton-ish look and accidentally joins a tournament where he wins and one of the demons takes him under her wing and has him kill a thousand people (or some.big number) this is all I can remember I can't fined it and I'm not sure of I read it on here or on mangasee