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Demons are incredibly powerful and often volatile beings. They're known for eating just abut anything, though not all demons are like that.
They do not have proper names, but will often give themselves one after becoming a familiar to a Demonmaster. Without a Demonmaster, they cannot cross from their home realm to the human one, nor can they return home.
Unfamiliar demons are not good. Strong demonmasters can keep any demon in check, with help from their familiar, but when a weaker demonmaster attempts to summon a demon, it has the chance to kill him or her and will proceed to run amuck. These rogue demons often must be recaptured by a more experienced demonmaster and returned to its realm.
No one besides demons know what their home realm is like, and none will tell any but their masters, but the general impression is that it isn't exactly a good place for humans to be.
Demons have two forms, generally, though some have three.
The first form is human-like, though they generally stand taller than the average person. They have talons and horns in this form; some retain their wings, but most do not. This does not appear to be their natural form.
The second form is, to say the least, terrifying. They are more animalistic, with bodies, heads, and front legs like lions, but their back legs, rump, and tail are scaly; they also have scaly wings. Their tails are usually forked, but may also have barbs or small fins at the ends. Males and females have manes like lions, though females' do tend to be slightly smaller. Females do have bigger and more elaborate horns, which come in many different sizes and shapes. A curled ram's horn seems to be most common, but others have also been seen, as well as one female having more than one pair of horns, though the second or third pair is always smaller.
For those that have a third form, it is somewhere between their human and beast forms, similarly to a traditional werewolf.
No one has ever seen a child demon, as they are very protective of their young and claim that the human world is bad for them while they are developing. It is also impossible to know the age of a demon without asking, and they consider it a very offensive question. Demon culture is a complicated thing, and humans are not privy to many of their secrets.
Demons cannot return to their realm if they are in contact with a certain very complicated, very expensive alloy of metals, which is a fact known only to demons and to Demonmasters. Because of this, Demonmasters have sets of these rings made and enchanted so that only they can put them on or remove them from their demon familiar. This, however, can become an issue if the master is separated from his or her demon.