In a barren mountain range in northern Europe is a secluded cave.
Deep inside is a wide chamber that appears to have been used in ancient rituals. The walls of the roughly circular space are densely covered in cave paintings. At first glance, they seem much like those found in other caves in that part of Europe, rendered in red and brown ochres.
Look closely, however, and you will see that the most common figures depicted are subtly non-human. They are humanoid, but slightly hunched, and while it is hard to be sure owing to the primitive style of the art, each figure appears to have two short, conical horns on its head, and a hint of a tail.
Look more closely still, and you will find some more traditionally drawn human figures. They are being hunted by the others, run through with spears, battered with stone axes. In some scenes the horned figures appear to be roasting humans over open fires, or feasting on human-looking limbs.
The paintings are disquieting, but they are just old pictures. The scenes they depict happened countless years ago, if they even happened at all. A determined traveller seeking shelter from a blizzard might decide to ignore the paintings and make camp. He may even light a fire.
A well-used firepit lies at the centre of the room, slightly raised on a natural mound. Should a fire be lit there, wavering shadows will be cast onto the painted walls. For many, seeing their own silhouette superimposed on those eerie paintings will be the last straw, driving them to try their luck outside with the snow.
Others will stay.
Sometimes, nothing will happen. These lucky travellers will emerge the next morning, haggard from a night filled with half-remembered nightmares, anxious but alive.
Other times, shadows will move on those walls, shadows cast by nobody present in the room. The wavering firelight will blur their outlines, make them hard to pick out, and some observers may believe them to be a trick of the light. Watch closely, though, and you will see that the shadows resemble hunched figures, perhaps with small horns, and maybe a hint of a stubby tail behind them.
These figures will gravitate toward the shadows of those who are taking shelter in the cave. They may appear to stand over their sleeping forms. They may appear to be holding weapons, like spears or rough axes.
Often, a lone traveller sheltering in the cave will never be seen or heard from again. Their fate will remain unknown.
However, those travelling in a group will be woken by screams in the night. They will look for the source of the screams and find that one of their number is missing, their bedroll empty. If they happen to look at just the right place on the wall, they may see the wavering shadows of figures who are not present in the room, or at least not visible.
The fire will be low and red, so the shadows dim and soft-edged, but it will appear that a struggling form is being speared and clubbed by multiple assailants, and then dragged away, out of the circle of firelight.
The screams will fade slowly, seeming to sink into the rocks themselves.
And is that a new cave painting on the wall the next morning? Surely not - the ochre it is painted in is just as old and dry as all the rest. No, no, that picture of a man being gutted by horned figures must have always been there.