Night Hags

Overview:
In the less-travelled parts of Erden, in the fetid bogs, the brooding forests, and the windswept heaths, an ancient power lurks. Grim-faced elders pay homage to it, leaving offerings of blood sacrifice beneath twisted trees. Others lead witch-hunts to drive it out, burning herbalists, elves, and any others they deem suspicious in a misguided attempt to secure the outlying villages. 
This power is the race of night hags: ancient and capricious forest spirits taking the form of wizened crones. Night hags are immortal beings with the power to shift between the Ethereal and Material planes at will. These lovely creatures siphon power from the spirits of mortals, with children being their favored prey. Those unfortunate enough to spend any length of time near a hostile Night Hag coven will also be afflicted with horrible dreams so severe as to drive the victim to madness. Thankfully, night hags rarely strike out from their territory and have no desire to form any kind of organized society outside of their covens. Instead, they are greedy and territorial, defending their lairs and hoarding macabre ornaments and petty trinkets within them.
Covens:
Night hags form tightly nit covens in remote regions, each consisting of no more than a handful of "sisters". These covens are often found in the depths of haunted forests, bogs, and ancient ruins on the edge of civilization, ideally within reach of an isolated village or two. Despite their outward appearance, they are not uniformly hostile. In fact, many night hags actively welcome certain mortals into their families as witches, especially women cast out from their villages on suspicion of magic use. These pitiable "children", spurned by their true families, find a different sort of clan awaiting them in the night hag coven. Of course, the line between child, servant, and plaything is blurred to a night hag.

The Mobius Witch
Tortured, sacrificed, and then raised as an undead, mobius witches are undoubtedly the most fell form of night hag. Though their mere presence is enough to imbue mortals with a sense of impending doom, their physical form is even more disturbing to behold. They most often appear as a ghostly, pale woman with jet black eyes, a gaping, black maw, and long, tangled black hair obscuring most of their face. Their movements are unnatural and impossibly swift, often crawling on all fours with joints sticking out at strange angles, only to disappear and materialize behind a foe.