They are a race of small energy beings. While most do appear in the form of a transparent red generic humanoid with horns and bat-like wings, the precise form varies. They also vary in shades of red and brightness.
A Demon is unable to do physical harm in the real dimension. However, it is capable of possession. Each entity adds their mana to their host’s mana pool and increases the host’s force rating and grants the possessed the ability influence others, gnosis. Only some have other abilities. A rare few have multiple powers. Demons value power.
Most only have a magic force rating of one. Over time they do increase.