There are few things more frightening to the people of the West than Elves. Perhaps demons, who have the power to steal the souls of humans, are more terrifying but if so only to a marginal degree. The Elves and their fairy kin lair in deep forests across the world, and that includes the Dark Country.
In the forest of that strange hinterland they compete with Orcs over "food." Or perhaps they don't. Perhaps they share some common and diabolic cause, after all the White Lady employs those bloodthirsty, porcine beasts.
Elves and fairies come in all shapes and sizes. Some are roughly human sized but are incredibly beautiful to look upon. Others are tall leafy things with green, wrinkled skin. Even more have animal-like features, though they aren't as disgusting as Orcs. Some are small enough to fit in the palm of your hands, and others are as tall as trees. They are not one race but many races. They bicker and feud amongst themselves, but all are equally inimical to Humankind.
Despite this, they are not all aligned with Chaos. They seem to be much closer to the Old Gods: strange and alien creatures who permit humans to exist within their realms as long as the proper sacrifices are upheld. They certainly have a capricious nature, but they are not nearly as destructive as the forces of the Adversaries. Still, one would never confuse them for being "friendly."
Some theologians have postulated that they are the weakened forms of angels who refused to join the God of Law in his battle against the infernal masters of the Pit. According to this narrative, these angels were cursed to live in quasi-mortal bodies on the material plane, and now spend their days torturing the greatest of God's creations: Humans. Those few fairies who have been questioned on the subject simply laughed. Those few souls who have asked did not return to tell this rather enigmatic answer to those who would care. Of course, that's what legends say, and how can there be legends if no one ever returned?
Elves and Fairies have some connection to Changelings, though no one is entirely sure what the nature of that connection is.