There are too many of them.
This is the one pervasive, oft-repeated statement that holds consistently to be true. All other facts seemed to revolve and twist around this one truth. They can be killed, one at a time, but they always came back. You could run, but there are always more to find wherever you went. You could hide, but despite their glowing eyes they did not track by sight--it wouldn't do any good.
You could fight, but there are always too many of them. On close examination it would not be proper to refer to them as having skin, or fur, but they are covered in something black as oil. They moved sometimes like liquid, and pieces of their forms could tear off with attacks and be replaced in a couple of minutes; yet their teeth and claws are sharp, and there is strength and solidity in every blow they deal.
And those they kill did not merely die. It is horrible to watch, and surely moreso to experience, one of the black things tearing into a body, pulling something out that no physician has ever seen, and tearing it to pieces, causing a shadow to envelop the body and raise it up as another.
Rumors are inconsistent about their origins. Whether they existed before or not, it is said that bringing them into the world was fully the work of one very powerful mage, whose name is unknown. In conversation he is universally called the Defiler. They spread, from their first appearance, like a plague through the kingdom he lived in, and some say they consumed him as well. At first it seemed like a local problem, so nobody did anything. Then there were more, and more towns and more countries being swallowed up in no time at all, and by the time they were recognized as a real threat there were few nations still standing, none of them with the power to stop them.
By some miracle, there are still many people. Those who have always wandered the world, and a lucky few from nearly every settlement that has been consumed, still survive. A few fall prey to the dark ones every day, but only a few groups have attempted to set up shelters, and these led by the gods themselves. Those who believed that the gods could stop the dark ones were proven wrong when Eromus, the god of the sun, was swallowed up and became something even worse, even more powerful and destructive than several normal dark ones combined. Since then their numbers have dwindled as steadily as those of the mortals they sought to protect.
Now there are rumors of someone else. His name is unknown, but he wears white and is said to be able to move, at will, instantly from one place to another. He alone seems to know the secret weaknesses of every variety of dark one, as if he had trained against them his whole life; yet his first known appearances were sudden, and none seem to know him. He is on a quest that seems in vain, impossible: He wishes to bring everyone who still lives, every god and wanderer and refugee, to one place, and build a city, fortified with walls and many protectors, one that can survive any attack by the dark ones. Rumors of the man in white are not enough to bring hope; he is not the first one to have such an idea. Yet the gods he has met so far have thrown their support behind him, as if in some secret agreement or pact. Whether his plans will succeed, or lead everyone who is left into a trap, remains to be seen...