One's devotion to evil does not merely end with death. No, those whose pitiful lives revolve around their worship to dark deities often find themselves unable to "move on". . Upon death, these "true believers" transform into ghoul-like creatures that continue the pursuits they had in life. True believers are usually found in the ranks of cults, acting as fodder for their higher-ups.
A vile is a beast that's been transformed by the hideous magic of an evil spellcaster. The creature takes on a new form, that of a winged reptile with a despicable bent. Viles are typically used to hunt for the spellcaster's nemeses over the course of a short time period, usually no more than a single evening, as sunlight reverts them back to their original forms.