Before Zach went to fight the angel in the castle, he had summoned his army of the undead and asked them to not interfere in his battle. Of course, Cerberus was against it, but Zach had different plans.
"Cerberus, I want you to do something else in the meantime. I have been talking about ascending for a while now, yet, doing nothing. I don't want to stay here long. I will surely leave tomorrow, so I have to finish up some unfinished business."
[What is that, my liege? Just tell me, and I will risk my life to complete your task!] Cerberus said with his utmost loyalty to Zach.
"When Noah and Elina became my followers, I promised them something that the second realm is my realm, and there will be no evil deeds, but I can't be sure of that. However, I will have to ensure that, so I want the lesser-ranked demons to spread across the second realm and kill every bad guy you see.