Eren was battling with another demonic ranker at this point.
His battle had been shifted to the inside of Bulal's main base. He and a few other rankers forced their way inside the giant hall he was used to conducting his blood rituals in.
The rankers were sent to see if there were any survivors among the people Bulal had kidnapped from Pointlet Town. Since they were winning, Kiara decided to secure the kidnapped mortals and ranked guards if she could help them.
The demonic ranker Eren was battling with seemed more interested in finding his way inside the giant hall than engaging with his opponent as well. He soon found what he was looking for.
There was a red runic array at one corner of the hall. The array would usually remain inactive and invisible. But someone had used it not too long ago when the chaos was just starting to unfold.