Greetings,
This is about one of the most fundamental aspects of the cobha of Ithelak (and the other two worlds as well, though there are differences).
Spiritual beings regularly manifest themselves in physical form.
They do this at great cost to their energy, and although they can have multiple manifestations 'in store' to use, each one uses energy, so only the strongest can actually maintain more than two, and the weakest generally can't even manifest at all (except in slight influences on natural forces and whatnot, or in thoughts and suggestions in humans). If one manifestation is destroyed, they must exert more energy to develop another, and this takes time (along with energy).
This energy isn't well understood by mortals, but think of it this way: immortal beings aren't either dead or alive, but rather, their life is in a gradient form without ends. It is a gradient between two infinites. Only God is fully alive, and no one has ever become fully dead. Various things give life, other things give death. Manifesting gives death.
This energy is not related to righteousness at all. So don't think this is some sort of analogy (i.e. all demons are deader than angels, etc.), it isn't. It is just a fact of life up there.
There are good spirits and bad spirits. They have free will, and a choice about which side they fight on. Just like people. But because they live in the spiritual world, things are more black and white, without all the confusion here. They know which side they are on, and changing sides is extremely rare (most decided practically upon creation which side they were on, and have stayed that way ever since).
When an evil spirit manifests, it is called a Zheighor. When a good spirit manifests, it is called a Wizard. Many times it is difficult to tell whether something is a manifestation or not, and even harder to tell whether it is a Wizard or a Zheighor. (The generic term for a spirit manifestation is Darhainiín, in Elvish at least.)
Most spirits have specific, dedicated roles in the world, given them by God. However, not all of these occupations are occupied by angels. No, these occupations are warred over among the two empires quite actively.
These occupations sometimes require manifestation, but not all. The vast majority of them are for governing the natural processes, such as the weather and the oceans. Thus, some cultures name the winds, and those winds actually do have personalities and even sapience. Others rule over nations, guiding and counseling and defending them. This is done through seers (mortal liaisons with the spirits), or through supernatural means (via the elements or chance events, generally), or through direct manifestation.
Other manifestations are less 'official', and those are either because of personal relationships between spirits and mortals, or because of the spiritual war.
In those cases it is easier to tell if a Darhainii is good or bad, generally (they can deceive you of course), by the form they choose.
There are only seven human races (tribes) on Ithelak. No other sapient beings are present, so no talking dragons or centaurs, etc., right? Wrong. Those are present, but they are Darhainiín, not mortals.
However, the kind of form is really important.
For example, a centaur is never good. Neither are werewolves. There are extreme exceptions to all these rules, of course, as Wizards must sometimes go undercover, but don't bet on that if you are facing a satyr leaping at your throat...
Dragons, in the traditional sense, were used by both sides, as were swords (Ël-Ërai and Kimensul being prime examples) and human forms.
Natural animals were generally Wizardic: Zheighor rarely used things that bore the indelible stamp of God's creation, if it could be helped. But counterfeits were more common in the case of Zheighor than Wizards (evil spirits pretended to be good more often than good spirits pretended to be evil).
Darhainiín exhibit authority over all nature, unlike the Lords of the mortals, who are bound primarily to one of the seven elements. But their authority is extended in a different way than mortals: they use mere thought and the Tongue of the Angels rather than the regular way.
They exhibit magnificent awe and fear in those around them, when they uncloak their nature and power. They are masters of any skill they put their hand to beyond the capability of mortals. They don't just have it, of course, they have to learn it, but their aptitude is hundreds of times greater. And they read minds and hearts.
So, any questions?
And it has a ton to do with the whole history and plots.