Demons are elementals of Dark, they either serve their own needs or Abaddon, however as with everything there are some Demons that have turned to the good side and go to fight against Abaddon. They are incredibly strong, fast quick and cunning. Some Demons have the ability to fly, but this is only the high ranking.
Demons can come in many different forms but all possess the ability to take on a perfect human form. There are no signs that said person is a Demon. However as with all things there are draw backs. When a Demon is in his or hers human form he or she will have limited access to their increased physical abilities, however they still have access to their magical abilities. Since Demons are part of the Dark they have mastery over Dark based magic.
Demons also have the horrible ability to posses humans and use them as vessels however the more powerful the Demon the stronger its vessel needs to be otherwise it will burn up and die. However when an Demon has possessed a human it can mate with a human to create a Half Demon.
There have in very rare cases been an offspring born of a Demon and an Angel which is called a Immortals.
Demons are one of the hardest creatures to put down for good, however like everything they have their weaknesses. They are weak to Light base magic and can find it hard to recover from blessed weapons.