This much is known.
But he didn't have the secrets of the Skaal and something like that doesn't sit well with him. The Skaal are a noble group of people who would gladly lay down their lives for their beliefs. Herma Mora knows this. He also knows that they would do whatever it takes to protect their beliefs and their way of life.
Do you suppose that Hermaeus Mora set this entire fiasco up between you and Miraak just to gain the secrets of the Skaal?
"Hook, Line, and Sinker."
Miraak; the hook
His return; the line
You gaining the ability to defeat him; the sinker
Miraak's return doesn't just threaten the Skaal, but their way of life as well.
I know it seems a little dramatic, but if you have all the parts- why not?
Just a thought.

