Demons will be presented as an optional faction, exclusively on the Necropolis continent. Let me explain why.
They are the second tier of evil. If Necropolis is like a plague, needing human flesh as their sustenance, then the demons' goal is the soul. They can latch onto one of the sins: pride, greed, gluttony… and amplify it.
Since there is no inherent evil on the human continent, and people are free from vices, the only place where demons can gain a foothold in the world is on the Necropolis continent. This is where evil has accumulated in abundance.