According to the Elders Scrolls wiki, they are actually a mutated giant tick, notorious for draining the blood of drunkards unlucky enough to draw them to them whilst unconscious, and leaving them cold and dead for the morning watch to find.
This naturally makes them awesome, and so, in need of a 5th Edition version.
|And as if by magic, a...Oh crap! What is that...? Ah...no...NO.....NOOOOOARGH!|