I wish it was something Naomi actually can't subdue through brute strength. Something they would need to deal with to reach a compromise of some sort. A monstrosity not unlike natural forces, so something that was not under the demon king's power back in the day. A primordial entity of chaos that won't go away and cannot be defeated, but will disappear for a while if certain conditions are met.
That being said, I fear that even if that was the case (which I very much doubt), the thing would just turn into a woman and join Reda's harem. But I guess that would be funny as well.