Apart from the Time Sprite's main body, creatures that transmigrated through space and time would be rejected by space and time according to their strength. The longer they stayed, the stronger the rejection, until they were expelled into the space and time flux by this space and time.
As a demigod, it would consume more energy to bring Yun An to transmigrate. Secondly, it wouldn't stay long if it brought it back to the modern era.
If he killed Yun An and turned it into an undead, he could erase its life force and consciousness and transform it into a pure puppet. Unfortunately, Chi Tong's undead space couldn't withstand the totem-level undead of the Mythical Race at the moment.
Moreover, that would be too wasteful. It would lose the original intention of being a sustainable resource.
