Ur-viles are mysterious, dark and unpredictable creatures, thus taking a rightful place among the most interesting of species in the Land, either regardless of their looks, or more likely because of them.
Ur-viles have no eyes, instead being furnished with a super-sensitive sense of smell. Presumably due to this, most of the front of their face is taken up with a huge pair of gaping nostrils covered in slime, with which they can apparently perceive the world far more keenly than creatures with eyeballs. However, there is a downside - you never want to meet an ur-vile with hayfever. Ur-viles' limbs are disproportionately short and their skin is black and non-reflective.
The ur-vile language is near-on uintelligible; it consists of a wet-sounding barking that can apparently only be described as "roynish"... well, we guess that this is the only word capable of describing it, since it appears over a thousand times in each Covenant book.
Ur-viles are not natural beings - alongside their cousins, the Waynhim, they were created in breeding vats by the Demondim and as such are entirely incapable of reproduction. No, they're not going to ever hear the patter of tiny claws around any of their dens. This lack of ability to breed explains three quite separate things:
1. The ur-viles' predilection for never bothering with clothes. Y'see when the Demondim created them, they didn't bother endowing them with any genitalia. A naked ur-vile is therefore about as offensive as an undressed Ken doll. As a side note, this complete lack of clothings means of course that the ur-viles don't have any pockets. However, despite this, they are very capable of secreting items within their own bodies. No, we don't know exactly where or how either - and frankly we're a little afraid to ask. Ur-viles have been known to pull out items as disparate as daggers and cups from within themselves. One can safely presume that they'd make expert smugglers, then.
2. The ur-viles' readiness to form themselves into a compact fighting wedge, with each row pressing tightly against the one in front of it. Let's face it, you're not going to want to do that if you're nekkid and do in fact have a full package. There'd be no telling what your uglies would be bumping up against.
3. The ur-viles' endless quest for fulfillment. Again, a complete lack of meat and two veg would upset most of us rather a lot and perhaps this is why the ur-viles endlessly seek a satisfactory meaning within their destiny in ongoing existential angst.
Ur-viles are highly lorewise creatures as has been said and are innately magical. Much of their theurgy is based upon the shedding of their own blood and moreover, they are capable of concentrating the combined power of many ur-viles into one fearsome sorcerous force - to be wielded by the ur-vile Loremaster who holds the place of honour right at the tip of any traditional ur-vile fighting wedge. This highly sought-after position is said to be much fought over by the ur-viles - presumably because, much as with sled-pulling huskies, unless you're lead dog, the view is never really going to be that appealing.
The ur-viles are also capable of producing a mystical beverage named vitrim. in an oddly synaesthetic way both smelling and tasting dusty, this arcane brew can serve as a serious stimulant to humans, giving them a major extra burst of energy when desperately needed.
Role in the First Chronicles
Role in the Second Chronicles
By the time of Second Chronicles, the ur-viles have obviously wised up that Lord Foul took advantage of their self-contempt and bent them to his will. He therefore is no longer to be trusted - gee guys, about time... the clue's in the name. With less of a role to play in this second trilogy, because Covenant and Linden soend so much time outside the Land swanning around on a cruise, the ur-viles' notable deed therein is to have created Vain, that enigmatic figure that they see as the culmination of their wyrd (or weird, or word or würd even - geez, Steve, make up your mind already). The ur-viles are actually wrong about this - see below - but Vain turns out to be A Good Thing™.
Role in the Third Chronicles
When we at last get to Third Chronicles, although remaining mysterious, the ur-viles have fully aligned themselves with the forces of good and are properly kick-ass throughout. There aren't many left by this stage - no baby ur-vile-ettes, remember? - so the remaining ones have banded together with their Waynhim cousins to do what they can to help Covenant & co. Their heroic achievements throughout the Third Chronicles are both many and legendary and are too numerous to list in full. However, chief amongst them are:-
1. Being minders to Esmer and doing their level best to lessen the effects of his schizophrenic actions. They also manage to shackle the raving loony and render him powerless at a very critical moment.
3. Chowing down on all the freed souls of scorned women released by She Who Must Not Be Named. This has the surprising result of a) all of a sudden making the ur-viles (and waynhim) a whole heap less fugly, and b) giving them a sudden obsession with horticulture, since they all then decided to become apprentice Forestals under the watchful eyes (or rather, lack of eyes) of Caerwood ur-Mahrtiir. This is clearly the ultimate solution to their destiny, the final meaning of their wyrd (or weird, or word or würd, or whatever the Hell it is).
Ur-vile hotness poll
Where to find ur-viles
Not too many of them on the Internet, and definitely not many enough, but more than some could expect. Mostly on DeviantArt and Kevin's Watch.
http://zorm.deviantart.com/art/TCOTC-Translating-ur-viles-WIP2-318159360 (tthe same as the previous one - just in case, and because more can appear later, before someone updates here)