For as long as most can remember, the dead have not gone quietly to their grave. Each race and culture has a way of dealing with the dead so that they do not return. Any corpse not so tended has a very good chance of rising again as one of the Fell.
The Fell, when they first rise, still look much like the corpse did, and from a distance, may not even look particularly monstrous. If someone died in their sleep, or quietly, the resulting Fell might simply look ashen and a little dazed.
These tortured walking corpses retain some of their previous intelligence and skills. The spirit is trapped in the body, but the mind, apparently, takes some time to fully die after it is reanimated.
Unfortunately for anyone who encounters the Fell, they burn with an insatiable hunger for the flesh of the living. They will only eat living flesh, in fact, and vastly prefer humanoid flesh to any other.
Because they are already dead, the Fell are harder to put down than normal people. They are also significantly stronger than they were in life, and they never sleep, and do not need water or food or even air to sustain them. A Fell who gorges itself on human flesh can go for months without eating again.
Once the Fell see or hear or smell their prey, they hound it relentlessly until they are stopped or until they finally catch up. The Fell are tireless, and because they no longer need blood to flow, even horrific wounds will only slow them down. They must be physically dismembered to be stopped.
As the Fell, also known as Ungrals, age, they grow in strength and dark will. They become even more implacable, even harder to kill, and resistant even to mind-affecting magic.
No Fell can heal naturally, but the Legates of Izrador are able to channel unholy power which binds them back together. They are also adept at controlling them, and the Fell are often used as blade-fodder in the relentless wars against the Dwarves and Elves.