Demon: An evil spirit. A source or agent of evil, harm, distress, or ruin. A supernatural and often malevolent being prevalent historically in religion, occultism, literature and mythology.
This is the Merriam-Webster definition of demons, however, it's interesting to find out that demons didn't start off as solely malevolent beings akin to those in contemporary movies and fiction (Cambion Rider Chronicles included but we'll get to that in a moment).
For example, the Ancient Egyptians looked at demons much differently than they are viewed today. Demons were divided into two classes, guardians and wanderers. Guardians centered around specific locations and at times showed benevolence. Some protected entrances to the underworld and prevented human souls from sneaking into paradise.
The wanderers at times served as executioners for deities doling out punishments as requested by the gods. They are also the demons that are believed to cause mental illness and take possession of humans or enter their bodies at night and cause nightmares.
How do Demons exist within The Cambion Rider Chronicles
These ain't your Ancient Egyptian demons. They are not wanderers or guardians but evil creatures trying to bring hell's party to earth.
It may be obvious, considering the series centers around a cambion, that demons will come up fairly often throughout the novels. So what to say without saying too much and giving the game away. Demons exist in a hierarchy structure with seven crown princes of hell. Beneath them are the dukes and earls and so goes the chain all the way to the imps and harpies. aka demon peons. Think of it like a corporation that is governed by a board of directors.
Different demons have different abilities (clairvoyance, telepathy, etc.,). Depending on how far removed they are from the princes, the strength of those abilities ebb and flow. So what keeps the earth safe? Seals keep the more powerful demons locked in the underworld. That said, seals can be broken and once they are, those same demons are free to make a hell break.
Minor demons are quite literally the devil on the shoulder. They influence humans to give into the dark thoughts in their heads, and as humans give into their vices, their corruption feeds the demons.
Wanna learn more? Read After the Flesh.