When old-man Joo Seo-Cheon dies, he is returned to his eight-year-old body and is given the opportunity to change history and perhaps prevent a war (I say "returned" and "given," but as far as we know there is no agency behind his soul migration). He uses knowledge and discoveries from the previous timeline to help advance his goals. He knows the locations of treasures and cultivation ingredients which will be discovered later and accumulates great strength and rare techniques (learned from martial arts tomes). A lot of the manga is him doing these treasure-hunting expeditions. He also makes crucial connections with people he knows will be important later on. Probably most important is Lee Ue-Chae, who becomes an extremely influential and powerful merchant. He also befriends/recruits a genius child inventor (Seung-Kye), a poison user (Dang-Hye), and an exceedingly powerful swordsman (Wu Qu). And he impresses many sect leaders along the way.
One of the faults of the way the story is presented is that he will have a goal "A", but then he must achieve "B" before he can do "A" and he must do "C" before he can do "B" and so on. That layering of goals along with the inclusion of subplots makes it so you can forget why he's fighting such-and-such group or what the purpose of a given quest was. I especially felt this way whenever he was fighting as the Phantom Bow Swordsman or as the Silent Reaper. Yes, he gets several nicknames as he travels: Killing Noise Blow, Phantom Bow Swordsman, Phoenix Fall, Pure Blossom Sword, Dagger Ghost, Silent Reaper. For what it's worth, up until chapter 53, his goals involved finding treasures and ingredients to increase his strength. After that point, it seems he's starting to focus on undermining the Dark Heavens faction, either by thwarting their plans or killing their high-ranking members.
This world is split into many factions, and their interplay isn't always clear and often seems that new ones are just made up as the author goes along. We start out with two broad groups--the Just Faction (also known as the Murim Union) and the Unjust Faction. But then they tack on the Dark Heavens faction, who work in the shadows and only Seo-Cheon knows of their existence. They are the ones who caused the all-important war. Then later, there's the introduction of the Five Evil Sword Sects, the Five Great Royal Families, and the Two Demonic Forces. And to further confuse matters, the Unjust Faction is later translated as the Unorthodox Alliance and the Just Faction becomes the Orthodox Sects (or the Martial Alliance) and may also be synonymous with the Beggar Sect and the Nine Great Sects. And finally, the Black Way/Xia Wu Sect is tacked on as a new category.
[Reviewed at chapter 110]