Two hundred thousand years ago, a nation of warriors built a city upon a river whose name has long since been lost in time. Such is the weight of years, even, that the river itself has disappeared, leaving only a blasted wasteland full of cracks and fissures that belch poisonous smoke. But even though the river, and its city, are gone, those warriors have not completely disappeared. One of their champions, the best of the best, has never died, in all those millennia. All that is left of him is a giant flying head, with wings and snakes for eyes and a pair of dangling, gangrenous arms, each one bearing an ancient sword made of bronze.
This undead thing roams the blasted plains at night, preying on those unwise enough to venture through them. For the blasted plains are full of toxic smoke whenever the sun goes down. As the earth cools, away from the sun’s glorious light, all these fissures belch forth the smoke that eats away at the minds of men.
Or that’s what you read in the book. You’ve never been to these lands before, and the locals all stay out of those plains, even during the day. They are just peasants, what do they know, really?