Heroes of Might and Magic II: "The Necromancer's Invasion"

Demons will be presented as an optional faction, only on the Necropolis continent and nowhere else. I'll explain why.
They are the second tier of evil. If Necropolis is like a plague, they need human flesh; it's their food. The goal of demons is the soul. They can latch onto one of the sins: pride, greed, gluttony… and exploit it.
Since there is no evil on the human continent initially. People are free from vices, so the only place where demons can gain a foothold in the world is the Necropolis continent. That's where evil has accumulated in abundance.