You can somewhat ignore the other reviews. This is a very good murim manhwa, even if the premise is cliche (the usual OP martial artist reincarnates into a weak body with a horrible reputation). There is A LOT of character development, and not just for the MC but even the side characters and the heavenly demon. The art and character designs are phenomenal and the heavenly demon looks absolutely divine. The pacing is actually quite nice, he becomes strong fairly quickly due to early timeskips. I think the main problem this manhwa has right now is the lack of a powerful antagonist and the lack of presencethe MC's past life has. It's been 149 chapters but the MC has only used his assassin skills once, and that was when he hid his presence in one of the earliest chapters. I don't really get why the author made the MC into an ex-assassin if he was never going to use any assassin techniques (he doesn't even use hidden weapons)
Overall a very good murim manhwa that focuses heavily on character development and world building but still has a lot of action.
The romance will probably not be good since the MC is around 70 years old mentally and will probably marry someone young enough to be his granddaughter, but the series is still nonetheless an S-tier murim story.