A Demon is simply a human who has spent an extensive period of time being tortured in Hell, which as a result has stripped them from their humanity. They are constantly deployed/ produced in Hell, with the most frequent type being that of a Black Eyed Demon. Other eye colours are very rare, and rarely seen.
Human Life
All Demons were once human, hundreds of years ago. During their lives they must have met the criteria worthy of Hell. This could either be by leading a particularly sinful life, or through witchcraft or selling one's soul to a Crossroad's Demon.
Time in Hell
Within Hell human souls are tortured for centuries for the purpose of creating more Demons. Time is warped in Hell, for example 4 months in Hell could feel like 40 years. Therefore the time a soul is tortured for is extrmely lengthy and seemingly never-ending. Depending on the individual, a soul could eventually be offered to end their own torture in exchange for torturing others. Once all humanity is lost, new demons often occupy positions under more influential, powerful Demons. This could involve work either in Hell or on Earth. Even if it is their 'home', Demons still consider Hell to be a prison, and therefore seek to escape.
Escape from Hell
There are numerous ways a Demon could escape from Hell. More powerful Demons, such as Azazel, Alastair and Lillith, may be able to move freely in and out of Hell. Weaker Demons, however, are trapped unless otherwise.
One means of escape are through Hell's Gates- portals from Hell to Earth. Although normally locked, they can be opened. Normally the Gates are guarded by a Devil's Trap, or other means to prevent other Demons opening them. However humans have often been manipulated by Demons to free their friends.
Other means of escape involve specific summonings, often done by witches but can also be performed by novices with access to black magic.
Return to Hell
Normally once a demon is exorcised from their human body they are instantly returned to Hell. Demons can also be completely destroyed with certain weapons that have been created by humans, although they are extrememly rare.
