"Such worlds become akin to Hells themselves, where cruelty and suffering is the norm. Societies fall apart or turn to empires of pure evil, as if out of exaggerated fantasy novels. But real. The greedy and selfish and cruel are tempted to ever greater greed and selfishness and cruelty. Senseless wars without end, slavery from birth to death in constant pain, and far worse. Thankfully, this is rare. Most worlds are sealed against demons, or still in contention, and only a bare few fall completely. Demons do this with possession of the unwary, with poisoned gifts of power that cause corruption of the mind, and with mundane ideas, lies, fears, and temptations. Some of the souls which Hell claims, we know, become Demons with terrible powers and without the capacity for redemption. There is nothing left but evil. They are more weapons than people. We have tried to redeem them, it does not work. The fate of the rest is unclear. There are perhaps on the order of billions of demons, though typically only a few thousands at most active in any single world. Hell seems to have some limit to how much it can intervene, much like Heaven does. I do not expect you to take all these worse as unvarnished truth without checking, but if you try to compromise with Demons everyone will regret it. Please be wary."