The Sect Master has one wife and 4 concubines and among those 4 concubines, Xu Yin, is already given to Bai Lung as a slave.
All while the others were successfully convinced to spend a night with Bai Lung through emotional manipulation, while two concubines showed a strong disagreement with this, one particular concubine was happy about it.
After all, she must have wanted a huge snake to enter her hole and dig deeper. The mystery of women, even Buddha failed to fathom it.
It is unclear what's going on in the Sect master's head as he was forced to take such methods of letting his concubines sleep with Bai Lung, however, some questions are left unanswered.
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At Night, in The Sect Master's chamber.
All afternoon I did nothing aside from thinking about my life choices
Bai Lung thought while he was somewhat excited to sleep with four mature women. He is but a man with need and will.