So, a human is pretending to be the demon lord (something that actually never existed before) and creating either modified demons who can pass for humans in human society or modified humans who think they're demons. On the other side, the utterly overpowered idaten (well, one of them anyway) can actually just wipe them out of existence any day of the year but aren't motivated to do so. At least two of the idaten have cooperated with humans or taken over a human nation and are experimenting to try to figure out the nature of demons and idaten.
Given the massive disparity of power between demons and idaten, I have a feeling the demons aren't the real bad guys here despite their seemingly evil attitude. They're just too ignorant.
Also, Prontea is a creator goddess able to manifest ANYTHING, including technology, from thin air. Right off the bat Over M is already outclassed.