It's not just Xiaofan's hope of regaining his power, it's also his last hope for survival.
If he insists on going, and no one can stop him, then having power to protect himself might give him a bit more chance of survival.
Here, Ye Aotian was rushing non-stop towards the Ren Family.
How did he not realize it before? Back then, eldest sister-in-law joined Yanhuang Soul during that time and quickly became close with Ren Zhongtian. Initially, Ren Zhongtian and Cui had a good relationship; it turned into a triangle only because of eldest sister-in-law's arrival.
Ren Zhongtian's mind was never on women. If eldest sister-in-law's identity was the Saintess, then Ren Zhongtian was very likely the one who saved her back then.
The problem suddenly made sense. Really, but if eldest sister-in-law was the Saintess, why didn't she reveal the Miaojiang Secret Technique and see if Xiaofan could use it to recover his power? Could it be that this Secret Technique is useless?
