The Gaurim are the fallen 'angels'. They aided and abetted the humans in worshiping them (the Gaurim), which had been expressly forbidden by Eos.

Over time, as they fell more and more, they corrupted the light of Eos that shone within them and twisted that light so that it was no longer a pure white, but many darkened and evil shades. The greatest among them, whose name had been Mauril but was changed to Erag - the light that was in him was so twisted that it was a brilliant darkness that shone out from within him.

After the fall, the Gaurim's physical manifestations (light-filled, perfectly beautiful, larger-than-life, genderless, humanoid bodies) were destroyed and the Gaurim were cursed to became purely spiritual beings. Only Erag, the greatest among them, who led them in their fall, was given a physical body - that of a great and powerful serpent.
The Gaurim desire always to have once again physical bodies, and this leads them to possess humans who open themselves up to such demonic oppression.
Erag himself (who really deserves - and will at some point probably get - his own topic) cannot possess a human in the same way as the other Gaurim because he has a physical body. However, he can do a lot of nasty stuff to humans, which I'll enumerate on the aforementioned, yet uncreated topic.

Suffice it to say that the above is a rough definition of the Gaurim.

Areth,

Ka

Are the humans who are possessed given any supernatural abilities after they are possessed?

Yes. Humans who are possessed by a demon (one of the Gaurim) has a lot of control over the elements - ie. they are, in the eyes of a commoner, magicians/sorcerers.

Supernatural powers probably include heightened senses (this leads to faster aging, btw). Also one would be able to sense the location of other people without any sensory perception - because the demon knows where the person's soul is, and can communicate that location to the possessed person.

During times of extreme possession, the person has very little control over what they are doing, and has extreme strength and all over sensory perceptions are much greater than normal. Because the demon is a spiritual being, it has no sense of fear or physical self-preservation, so demon-possessed individuals often die during times of extreme possession, or are injured very severely.

Beyond that, I haven't worked all the details out. If you have more questions please ask. It will only increase the total body of knowledge.

Areth,

Karthmin