A Nachtmacher is a terrible being that appears in a Schwarzvald after it matures and the animals die away.
Nobody is certain what a Nachtmacher actually looks like. Many believe them to be humanoid, and those few who have fought them in hand-to-hand combat and lived have suggested this as well. The reason for this lack of information is that the Nachtmacher is surrounded constantly by several yards of absolute darkness, impenetrable by any means. The creature itself can see just fine, of course, and the dark of the night is equally no threat to it. The Nachtmacher is a predator by nature, but it is intelligent, sharper than any animal, and fabulously cruel.
Aside from the darkness that surrounds it, these sinister creatures are known to be able to mimic human voices, mannerism, and even personalities with uncanny accuracy. And they always seem to know what to say to disturb, trick, or bait the humans they stalk.
They are used as bogeymen in the Kaselreich to discourage gossip. Children are told that the Nachtmachers are invisible, always there, listening to everything they say so that someday they can use it against them. This may actually be true.
Nobody has ever seen a nachtmacher fight, of course, but the corpses they leave tend to be twisted apart, as one might twist the legs off of a roast chicken. They seem to be very strong, but also precise. Each Nachtmacher has a different piece of the human body it prefers to take, whether as food or trophies, nobody is sure. It will leave those missing that piece alone.
It is commonly believed that one can repel a Nachtmacher in any of several ways.
First, though they keep darkness with them. the Nachtmachers never seem to strike during the daytime. They are creatures of the night, and perhaps they have no power when the sun rises.
Secondly, the Nachtmachers have certain rules, directives that they follow regardless of their utility in securing human knucklebones or tongues, livers or eyelids or whatever it is they favor.
For example, everybody knows that a Nachtmacher cannot enter a house (though what constitutes a house isn't clearly defined) unless it asks a riddle that the occupants of the house cannot answer. A correctly answered riddle renders them safe from the Nachtmacher for the night. An ignored or mis-answered riddle makes them fair game. It is this feature that grants them their most notable literary appearance in The Nine Riddles of Father Dietrich.
Another piece of confirmed lore is that a Nachtmacher cannot bring itself to move or touch any quilt sewn by a mother's hand, not even through the use of instruments.
Other proofs against the Nachtmachers abound, some true, some painfully false, and peddlers in charms and wards against the creatures (some of which even work) can be found anywhere where the brush has gotten a bit too thorny lately.
Finally, whatever their power, they are corporeal. Even with "permission," a Nachtmacher could not, say, squeeze beneath the crack of a stout door, or come up through the narrow drain of your privy or bath. That would be an Aufhocker.