Why did the mist sickness only happen after the Lord Ruler’s death?
It didn’t. It just happened on a much smaller scale. As you remember, the Lord Ruler basically meant stagnation. Because it seemed the Lord Ruler would be taking the power again (as was intended, and as apparently had been done many times before), and because of the extreme stability of the Final Empire, Preservation (though it really only had a shadow of it’s mind left) wasn’t as freaked out. After the LR died, Preservation began to attempt to create more Allomancers for the reasons mentioned in question 7. It also left clues, such as the number 16 everywhere, so that people would know it was Preservation doing it, and not just random chance, or ruin. Turns out that that didn’t work so well.
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