A wretched entity of the frozen lands, the WinterKill is the remnant of a mortal left to die in despair and endless cold.
There are many dangers at the poles and in the icy mountians of the world, but few dangers are harder to survive then an attack from an Ice Ghoul, a shadow-spirit whose very touch can be deadly.
A plague has hit the local area.
In humans it affects only the most vulnerable, the children and the very old, and even than it's little more than a summer cold. 24 hours of sniffles and then it's gone, barely noticeable really.
To sheep however it is fatal and the whole economy of the area is in serious trouble. If this keeps up the area could well be facing famine.
Somebody (enter the PCs) must find the rare herbs needed to make a cure.