The Yin Child is completing the missing parts of her body.
Yan Daozi and his old friend are likely the creators of the Yin Child, or they hold parts missing from the Yin Child, which is why the Yin Child killed Yan Daozi's old friend and why Yan Daozi is afraid.
This is also why the Yin Child appeared in Black Mountain Village.
This has related evidence, as Kou Yushan once mentioned that during the Corpse Nest incident, Yan Daozi acted, causing corpses to be torn apart out of thin air, an ability similar to the Yin Child's power to rip off arms.
In addition, as mentioned in the Ming Zhang Notes, the yin object comes from evil spirits, giving Sang Que reason to suspect that Yan Daozi's ability to dismember corpses is connected to the Yin Child.
Thinking of this, Sang Que immediately recalled her two visits to Yan Daozi, both times feeling a longing for the locked room on the left side; it was not her desire, but the Yin Child's.
