Heng Sao Tian Ya
I think they intended to use the fetal poisoning in order to hide his identity. They keep talking that being a celestial master teacher is big deal for the other worldly demons and they would do everything in their power to kill it before they can grow. Even the Zhang clan would be powerless if the other worldly demons use all their might. Hiding Zhang xuan and using the poison should be their best course of action. They probably have a divination that Zhang xuan would be back and marry the Lou princess a year from now that's why they force the marriage hoping the divination would come true. Just a theory though.
At this point, I'm not sure if I want to believe that ZX is actually from the Z clan or not. Because if his bloodline is hidden, it has to have been a rank 9 master teacher who did it, or ZX would've been able to see through it by now. But if it was a rank 9 MT who hid it, he should also have been able to cure him of the poison.
Even if Zhang Zuan seems like the young master of the Zhang clan, I bet you 2SS that he belongd to a higher and more formidable clan that would be revealed to be unique and unknown of and also just coincidentally have the ability to find treasures in their bloodlines that surpasses the treasure finding ablities of the Yun Clan by x3000%