The Mists Form
In writing this book, I had to nail down a few worldbuilding issues I'd been contemplating even before the first trilogy ended. What would happen to the mists, for instance, once Sazed took over and became Harmony?
The mists, obviously, are a big part of the series. It didn't make sense—either narratively or worldbuilding-wise—to lose them completely. However, they'd been created as an effect of Preservation trying to use his essence to fight against Ruin's destruction of the world. So . . . wouldn't they go away?
I decided that Sazed would still send them. They're part of the nature of the world now. To acknowledge what had happened, they wouldn't come every night any longer. But they would come. They were changed in that they are no longer simply the raw power of Preservation; they're now a part of Harmony—so they no longer pull away from Hemalurgy in the same way as they used to. They still have the odd effect of being able to power Allomancy. (And Feruchemy as well—if one knows how to do it.)
The mists are, in part, the raw power of creation. And when one is favored of Harmony, the mists have a greater effect than they might otherwise have. We'll see more of this later.