Demons are, to the average Atheian, considered to be a vile, chaotic fiend, summoned from Congeria by demonolgist wizards who use their evil magics for good. This is not always completely true, but sometimes the fact that magicians do enslave them drives them to evil. Of course, some are born evil. There is infinite variations between each demon.
Wizards, however, are not perfect. No one is. Sometimes they make a mistake.
Humanity might get lucky during those mistakes, and they'll occur during a summoning, or when orders are given, which will almost always directly result in the demon killing the wizard and thus going back to Congeria. Sometimes, however, humanity is unlucky.
A wizard may make a mistake when dismissing a demon back to Congeria. Once again, humanity may be lucky, and the wizard will get killed and the demon goes back to Congeria. Occasionally, however, when the mistake is made, the effects can be bad.
The process starts the same as in all the other mistakes. The magical enchantments binding the demon to follow the wizard's orders fall, restoring free will to the demon in question. Naturally, the demon does what any demon would do in a similar circumstance- kill the wizard. Usually, killing the wizard results in a dismissal, as the wizard's magics are actively preventing the demon from returning. But in some cases, the demon can't return. The wizard's mistake lasts beyond his death.
It is at this point that the demon is reclassified as a daimon. Usually weak from being trapped from Congeria, they thirst only to be allowed to return home. No matter how many humans need die to do so. There are three main types of daimons, though there is the occasional daimon who cannot fight into any of those categories (see The Circle of Culthus for an example). Likewise, though the daimons will usually be a minor threat as they can't rejuvenate themselves in their home, some were originally powerful enough to be a significant threat in their time spent as a daimon.
This type requires sustenance to escape their bonds. This sustenance is almost always the flesh of the race that summoned them. Typically, if a human wizard summoned the demon that would become a siren, the siren requires human flesh to escape. The amount depends on the siren, though a general rule of thumb is that the more powerful the siren, the more food it requires.
The way the siren acquires food is variable. Some use magically-enhanced musics (traditionally singing, but some have started delving into the world of musical instruments), and others use magical seduction. Though these two are the main ways, there are a variety of others, and are only limited by the creativity of the siren in question.
They are the strongest usually when unexpected, as they can quickly tempt and ensnare you in their trap. When expected, however, they are relatively easy to defeat. Knowing that they will sing or that they will attempt seduction allows one to make safe-guards. Which in turn allows sending in specially prepared soldiers. Those soldiers can then make quick work of the siren.
The siren is the most common type of daimon.
The Djinni, also called a Djinn, Jinn, or Genie, is a curious type of daimon. Unlike the others, which feed upon humans, they gain release through gifts. The most powerful type of daimon, the catch is that they can only use their power in the bestowing of wishes. The amount of wishes that a genie can grant depends on the genie, though the number is usually low. One to three is the most common, with four being all-but-unheard of, and five is next to impossible.
The Djinni is a very crafty daimon, and is very vindictive. They try their best to twist wishes into something harmful to the wisher. Of course, usually only the last wish granted by the daimon will be fatal, with the previous ones engineered to get the wisher into situations where more wishes are required to save themselves.
Though Djinni aren't normally found in something as ridiculous as a lamp, they do sometimes take up portable abodes, so that potential wishers can wish for things whenever and wherever they are, and thus free the daimon that much faster.
A common phrase used to install discipline in a child by their mothers is "a banshee will get you." Indeed, the banshee is the most terrifying of daimons, let alone anything else on Atheus.
The banshee is a soul-sucker. They require, like the sirens, sustenance to get back home, but instead of physical, tangible sustenance, the banshee eats a person's soul. No one, not even the banshee itself, knows what happens to the soul after digestion, which perhaps adds to the horror of these monstrosities. Most sciremagi and scholars agree that the soul probably doesn't go to the Afterlife.
What happens to the body of the banshee's victims? As they aren't actually touched, and the brain and the heart and all the other organs are still fully functional, the soul-sucked victim still lives, after a fashion, but lack all rational thought, personality, and that crucial spark of life. Most people simply kill a soul-sucked person, to put it out of its misery if its in pain ( they cannot communicate), or simply so that they have a body to mourn and bury.
Killing a Daimon
Killing a daimon is quite simple. Though some types are resistant to various materials and methods, in general all one has to do is chop them into enough pieces.
This is harder than it sounds. A daimon retains all magic and shape-shifting abilities (changing into a dragon is always a possibility) as before, and though most tend to fight in their natural form, for some that form is covered in sword-proof scales and tentacles.
Hitting a vital organ usually results in death. Of course, setting fire to the corpse is generally a good idea, as the daimon may be faking.
Death of a Daimon
The daimons are trapped by magic to stay in Atheus. As the magics that bound them does not halt with the summoner's death, they will not stop with the daimon's death.
As the body of the daimon is gone, however, the daimon becomes but a ghost, a spirit. It has no power, and no abilities. The fallen daimon becomes little more than a part of Atheus, and its consciousness spreads slowly, painfully, until the daimon in question is no more.
Sometimes, however, someone or something is born that is receptive to possession. The fallen daimon, if it notices this individual, can then use what little strength it has and inhabit that creature.
Inside this new body, the daimon lacks its previous powers and abilities. It is limited by the body its in for its strength and abilities. However, if in its new body it succeeds in its previous mission to free itself from Atheus (or something similar: in the case of the Djinni, if it helps a certain number of people), then it can release itself from Atheus and end the possession.