The four Demon lords and Ancalagon are both very near god level, they are essentially the highest tier of demi-god type creatures at least for demons. However they are still bounded only to physical manifestations, so while they are not as free as the gods themselves, they are still almost as powerful as the primals. (For example, Baalrog can hold his own against kilgarrah for a good fight, with a 40% of victory.)

And yes, as Lexicon stated were starting it off slow, the demon lords will be the master behind the strings of their armies that we will have to deal with. However some Gods might want to try to ally with the demons instead, for their own personal motives and to help inspire political aggression between the pantheon.