Zhao Yuanzhen's thought process was quite straightforward.
If you're having an affair with Mei Yingxue, I'll just drain all your energy completely, so you can't continue your affair with her.
On the other hand, if you're not having an affair with her and I've falsely accused you, then I'm actually rewarding you right now to express my apology.
In any case, whether it's a reward or punishment, she gets to decide, and the logic seems to hold up either way. It just shows how cunning and tricky the Demonic Sect Enchantress can be, totally unpredictable.
Yan Yu indeed couldn't guard against it.
First, it must be stated that if it weren't for the forced extraction of True Yang to assist others in cultivation, his physical fitness was actually incredibly robust; otherwise, he wouldn't have been a Mortal Warrior in his previous life.
