Their origins are shrouded in mystery. Some believe them to be the "Children of Satan" while the more commonly held belief is that they're Dark Fae who dabbled in the wrong kind of magic. In truth they are in fact on par with the Divine, counterparts to them.
In 1954, Suzanne got involved with a plot that lead to a demon named Garmr. She successfully stopped it. Her encounter was one of the first between an Unseen and Demon in over 500 years.
Witches call them "The Desolate Ones"
Half-Demons are called The Desecrated.
When Demons chose a disciple or follower they generally Mark them in some way. Often times a Cult will brand themselves with a similar looking mark to prove their allegiance.
List of Demons