Sidhe

The Sidhe are immortal beings that live in the Otherworld with the Allmother. There are hundreds of types of Sidhe, varying from faeries and elves to banshees and cait siths. Without the magic of the Otherworld, a Sidhe can become "dark". Dark Sidhe can be extremely volatile, usually violently so. They can only pass to the mortal realm through a tear in the veil between worlds, and when they do, it can be extremely disorienting and confusing making it difficult to find their way back before becoming dark.
Many Sidhe possess some form of magic. For example, banshees have the gift of prophecy while elves and faeries can manipulate plants and minerals. Other Sidhe, like the cait sith have great magical prowess and can manipulate nearly everything around them. When they become dark, their magic can mutate as well. While normally, a banshee's cry portends death, a dark banshee's cry can cause it.
A member of the Sidhe can also pass illnesses and diseases onto humans they come into contact with, most commonly, vampirism and faerie pox. 
If slain in the mortal realm, a Sidhe will not die, but will instead return to the Otherworld. Given enough time, their essence will become strong enough to regain physical form. If a Sidhe perishes in the Otherworld, their essence will be destroyed completely, and they will be unable to return.
The Sidhe can mate with humans creating, what humans call, Half-Elves. These children, being part-human and part-Sidhe, typically have physical and magical characteristics from both races.