This Swordsman Is Very Popular

A huge and underdeveloped living planet, with human dynasties, primitive tribes, mermaids, ancient battlefields of living corpses, and ferocious beasts everywhere... This is a world born to fight, and only the strong can control their own destiny. (This book belongs to the time-travel genre, and the combat power level will not be too outrageous. The important point is that the protagonist is not a sand dollar.)