Beisitian obtained the Shi Family's blood, specifically Shi Jianbin's. Why his alone? Because he was influenced by two kinds of bad water, making it more valuable for reference.
If they can treat Shi Jianbin in passing, it will be a great advantage for any upcoming battles, as the 'Jian' generation of the Shi Family is quite skilled with the fighting talismans.
Although there wasn't direct communication between both parties, Beisitian felt that a basic trust could be established. If it were other families or sects, such trust might not be so easily formed.
The next day at noon, Beisitian suddenly felt that the dosage of bad water in the food had nearly doubled. She was startled but continued to discreetly blend the food into liquid and pour it away.
In the afternoon, around three o'clock, just as Beisitian had begun to make some progress in her research on the two kinds of bad water, she heard a loud noise outside, like the sound of an engine.
