OK, these may seem unrelated at first but bear with me.

1. At the end of Winter's Heart Shadar Logoth and Mashadar were destroyed.

"All we've learned, really, is that Shadar Logoth is gone, and of that I can only say, the world is better off without that sinkhole of the Shadow."

2. On the other side of the World, souls rise up from the ground and become the living dead.

"But fear of the dead only explained so much. Maybe people were too frightened to think of washing, but it seemed unlikely that fear would take everyone that way. They just did not seem to care anymore. And weevils thriving in winter, in freezing cold? There was worse wrong in So Habor than spirits walking, and every instinct told him to leave at a dead run, without looking back. He purely wished that he could."

Doesn't sound related does it? Right, here goes with the theory.

Once upon a time a big group of people left So Habor in search of help, or on an adventure or something. Either way, this group of people stumbles across a city a long long way away from home. Maybe this city was still called Aridhol, all those years ago, or maybe it was Shadar Logoth, a place where even the Shadow fears to go. These people go in and find the city deserted, but they also find buildings in which to shelter for the night.

Nightfall comes...

Just imagine the scenario: One man wakes up in the night, thirsty or something. He stumbles out into the street looking for a fountain to fill up his waterskin. He turns a corner and sees a giant wall of fog coming towards him. Curious, he goes to investigate...and nobody ever sees him or his companions again...

Hundreds of years later, Shadar Logoth is destroyed by Rand and Nynaeve. So is Mashadar. And where do all these souls go? Back to where they were born. All over the world souls are being returned from the dead. Most of the people had nowhere to go - their cities were long dead and lost, so they move on. but some stay here and Linger - after all, thinks the group from So Habor, our town is still here. Let's go.

And miles away, the dead start rising...

Sorry it was a bit long, but it was effective, right?

Anyway, the souls released by Mashadar's destruction are free to wreak havok on the world.