Svartkogen
In dark times, a forest may be infested by some vile taint. This may be caused for several reasons. Perhaps a necromancer has made their home beneath its branches, their foul wizardry seeping into the very soil itself, spiritually staining it. Other times it may be some supernatural contagion, such as lycanthropy or vampirism, infecting the blood which spills onto the earth. In rare chases, an act of great evil is so potent that it leaves its mark irreparably on the trees around it. These afflictions affect the forest as a whole, the woods themselves growing sick with the defilement.
A svartkogen is a form of twisted forest spirit created by the forest itself for defense. Distantly related to trolls, these creatures are made to hunt the supernatural dangers that contaminate its mother and home. Vicious predators of speed and arcane aggression, they an are evil created to fight evil. They are able to smell and track all manner of monster for hundreds of miles, following the spiritual imprints left by their passing. They are able to shift their forms, allowing them to pursue their quarry even into human cities without arousing much suspicion. Those who look closely may see that they appear to have too many teeth, or their eyes are too big and strangely colored. But these misgivings quickly pass as their memories are cleansed, leaving only the impression of a passing shadow.
Once they have found their quarry, whether in the forest itself or after a long pursuit, they are swift and merciless in dispatching their prey. If their prey was a particular challenge to find or kill, or if they displayed other forms of power and strength, the svartkogen may eat it to honor them. Trophies are always taken and proudly displayed.
Yet if the svartkogen succeeds in eliminating the external evil from its forest, it faces banishment, for it cannon remain as an evil thing in the forest it was created to protect. Most do not survive being cut off from their creator, withering and dying like plants with no water. Those strong enough to overcome this separation are then doomed to wander the world, constantly driven to seek out foes and challenges to fill the void inside. They continue their war against those that would corrupt the natural world until they are slain in the process.
There is only one known wandering svartkogen, and many others likely exist, protecting the forests of the world.