99% sure the Denji holds the power of the leader of the fear demons. Full on Lich level shit he is beyond the simple demons of fear he I would guess is the demon of the end. That is why all of the demons have the same last memory of the sound of a chainsaw, that was just the demon of the end doing their job, ending things. This also explains the reason that there is so much interest in his "heart", his heart is literally a god-like existence if not full on god-like existence. Also if he is indeed the holder of the most powerful demon that explains why Makima (who is probably one of the fear demons) was there to find him immediately after his change. Currently he is not using really any of his power but, I would be willing to bet that if some real shit were to go down he would end them all.