If what Zhao Xiantong said earlier, "There might only be one ghost in the Ghost Realm, and the rest are Ghost Servants," is true, then it's just Ghost Servants. Li Yuan tends to believe this point.
And this Ghost Servant would transform into its living appearance, return to the Human World and bring those they hate, or those who killed them, into the Mass Grave, and if any innocent people were involved, those innocent would die as well.
And the newly deceased would come to the Human World, bringing more people into the Mass Grave, continuously, until the Mass Grave transforms into a Ten-Thousand-Mass Grave, a Hundred-Thousand-Mass Grave, or even more...
Fortunately, this Mass Grave seems to have only opened recently, and there are limited Ghost Servants walking out from it each time, initially only one, now it's two.
Before these Ghost Servants return, new ones cannot exit.
