Not much has been revealed about the inner workings of demons, except that there is a caste system, with lower demons at the bottom and older, more powerful beings at the top (sometimes referred to as gods). They must be summoned by a wizard or sorcerer using their true name to appear in this world, with two obligations to fulfill once done so: first, to attempt to escape whatever confinement may hold them, and second, to kill whoever summoned them. In appearance and ability they all resemble monsters, have some magical ability and are very strong. Like all creatures from the Nevernever they must create a body made of ectoplasm to exist in this world. If this body is destroyed, the demon is not killed but returned to the Nevernever. Despite their strengths and the intentions of the wizards summoning them, these demons are said to be uninterested in affecting the mortal realm on their own, if they even notice at all. The one group of demons that do care are called The Fallen. Generally, the Fallen are forbidden to interfere with humans by God. But sometimes they do anyhow. They also are allowed to interfere when called upon by a human.
Scions, like changelings, are the result of the mating of a demon (or other non-fae being of the Nevernever) and a mortal creature. These children tend to look inhuman, often disfigured or insane; even those who appear to be normal humans have an “inner” demonic form that can be seen by a wizard’s Sight.