Zimu fu (子母符) or Son-Mother seal is a cursed seal (符咒) that enslaves yaoguai, forcing them to do whatever their master tells them to.
The Curse[]
The yaoguai is forced to consume the Zi fu (Child Charm). The master could then burn the Mu fu (Mother charm), which connected to the Zi fu, drains their Spirit Power and brings immense pain to the yaoguai, forcing it to do their bidding. Also, the yaoguai are forced to consume the Zi fu each year or else they will suffer great pain.[1]
The 18th ruler of Yuyao once discovered a way to break the curse. But it has since been kept a secret in order to keep enslaving the yaoguai.
Donghua[]
Akuan and other yaoguai are initially enslaved via the Zimu fu. But Yuyao Princess falls in love with Akuan, ascends the throne, and eventually frees Akuan from the curse. She then tells him the secret to free the other yaoguai.