Wights are a type of undead created by attempts to bring spirits back into the world of the corporeal. Wights are a type of undead, their heart does not beat and they have no need of normal food or drink, but otherwise they seem almost completely human. They can even eat and drink if they'd like without the problems vampires tend to have.
However their bodies need to be sustained with living human flesh, mechanically this means that wights need to use the bite command on helpless PCs fairly regularly, inflicting one level of wound, none to mild, mild to severe etc. Those with regenerative powers offer significantly less sustenance.
If Wights don't feed often enough they will start to grow more corpselike, with some of extremities beginning to show signs of rotting and the rot slowly working it's way in before finally make it's way up to the face. Shortly afterwards if the wight continues to slip they will die, a wight that dies cannot return as a ghost.
Surviving as a wight is quite difficult, but still is the only shot at a second chance at life most people are going to get.