A “man-eating road” is descended from the wizard roads of old. When magic was at its peak in millennia past, a wizard could cast a spell creating a loop of road that took him and his entourage quickly over rocky or inhospitable terrain, towards water and, food and shelter, and away from harm.
While the wizards of old and their spells are long gone, these roads lived on without a master, reverting to their chaotic natures in the absence of a strong guiding hand. They sustain themselves on the meat, bones, magic and life forces of creatures that die while on them. They have taken to purposely attaching themselves to real roads, matching their stones or dirt, forking off in a new direction or lying in wait at the end of the road, extending off into the distance, around a bend or into the woods.
They range in length from 200 yards to several miles long. Like a möbius strip, they have no beginning or end, and as they lead a party away from the main road, they must detach themselves in order to create the road ahead.
These roads can be found out. A scout that leads the last member of a party by the length of the road will see the road extending into the terrain as trees and rocks part for the road’s head. If the road sees this about to happen it will try to dispatch the scout as quickly as possible, resorting to loose rock and trees if necessary.
Wizard Roads have 20 HD, the AC of an unarmored man and attack as 4, 8 or 12 HD monster when using trees and rocks. If the road is stretched to its full length by spreading the party along its length, the head and tail take double damage to cutting attacks. At this point, wizard roads usually try to negotiate.
They can understand any language, having existed long enough to hear them all. They can communicate by making letters appear in the dirt or scratches on the cobblestones. They will follow the letter but not the spirit of any deal they make, so they must be parlayed with as if they were genies.
I should acknowledge Danny the Street, an inspiration for this idea. He appeared in the excellent Doom Patrol comic and was created by Grant Morrison and Brendan McCarthy.