Rottereiters. Alternatively, Rottreiter, Rotreiter. All are correct.
The Rottreiter is, fundamentally, a form of undead. It takes the appearance of a man riding a horse. The two are inextricably bound – they can never travel further than a few yards from each other. Both are essentially the same organism.
The hooves of the Rottreiter are made of steel, and leave glowing red marks wherever they tread. Their manes are constantly ablaze, and their tails are long, streaming whisps of fire that seem to drift like silk in the wind, even when still.
The man himself seems normal, in many ways. Almost alive. Unlike most undead, they can still make expressions, sweat, breathe, and even talk. They also seem able to feel pain (though they never react to it in battle,) but the one thing they can't do is burn.
The Rottreiters ride in packs, dozens of them at once, tearing through the Schwarzvald wielding swords, screaming and trampling anything in their path. When they pass near humans, they usually ignore them (if they're in the way, they'll trample them without a second thought) but occasionally they will take notice. Once a person has been noticed once by a Rottreiter, they are marked forever.
If ever again that man crosses the path of the herd, he will be taken, and made to join the ghastly procession as it tears throughout the black forest. Occasionally, the Rottreiters will attack something – a town, a village, a caravan, etc. Their motives are totally unknown, but they can often be heard apologizing as they ride away.
There is some evidence that Rottreiters are persistent between Schwarzvalds. There are sightings of the same individuals centuries apart at different times.