"Ninety Ku's Magical Power now, but the speed of my magical power growth has dropped quite a bit."
Lu Chang'sheng wasn't the least bit frustrated—this was inevitable. The higher your cultivation, the more everything rides on your Immortal Dao Spirit Root Talent. Otherwise, the Spirit Realm wouldn't have so many low-level cultivators running around.
A D-level mid-grade Spirit Root Talent was considered middle of the pack in the Guangnan Domain. In theory, there was a chance to reach Foundation Establishment with it.
So, in other words, Lu Chang'sheng's theoretical max was Foundation Establishment.
Of course, that was just theory. The reason people in the Spirit Realm say that is because there have only been a handful of actual examples—just a rare few with that fate.
