Heavily recommend that you re-read chapter 16:

Heavily recommend that you all re-read chapter 16. One of my reviewers told me that it was stupid that Harkon was going to be getting incredibly strong just from selling souls. I agree with that statement. I want this to be a very, very slow power ramp. Not instant amazing god like power.

The deal he made with the 'serpent' was heavily changed. He will no longer trade souls with the guy, that was incredibly stupid on my part.

If you are a mobile reader, you should un add the book from your library and then re add it. There is a glitch where updates don't appear unless you do that. Anyways, little PSA is over. Another chapter will be out later today.