I think it all began when one desert tribe drove another from its water hole. The displaced people moved into their neighbors' territory and uprooted them in turn. And so it went, one tribe dislodging another, until eventually scores of thousands of square miles had been set in motion. […] As their numbers have grown, the circle has expanded until part of it lies across the Barrier in Perimal Darkling. There are rumors that the Wasters have mixed their blood with what crawls there in the shadows until many of them are barely human themselves.
– Marc, Dark of the Moon, "Chapter 3: Old Friends, Old Enemies"
The Waster Horde refers to the amassed tribes of the Southern Wastes, about 3 million individuals.
↑Dark of the Moon, "Chapter 15: The Killing Ground" — "The prisoner suddenly exploded into vehement speech that sounded like the yowl of a cat fight. He ended with a burst of scornful laughter, baring filed teeth. 'Ka'sa dialect, I think,' said Torisen."
↑Dark of the Moon, "Chapter 3: Old Friends, Old Enemies" — " 'Now there are some three million people down there caught up in it, circling, circling…' "
↑Dark of the Moon, "Chapter 3: Old Friends, Old Enemies" — " 'That was nearly three centuries ago, and it hasn't stopped since.' "