so I'm guessing you played dawn guard and after you blocked out the sun you started throwing party's ...nobody know there plans the only thing we do know is that they wanna block out the sun
http://media.tumblr.com/tumblr_lh9bj8Zty71qavolh.jpg

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Maybe I did play Dawnguard, or maybe I actually watched and read the interview that confirms the vampires want to block the sun because of this prophecy. Why do you think Harkon said "The Tyranny of the sun shall end"? Or why do you think Matt Carafano kept saying the same thing that Harkon wants to stop the end?
http://downloadxboxgames.com/2012/06/08/e3-2012-the-elder-scrolls-v-skyrim-dawnguard-preview/
"Dawnguard with a new proposal is made ??to you in the beautiful world of Skyrim: join the Guards of the Dawn, of traditional vampire hunters, or, conversely, become a vampire, but not a puny little vampire, or rather a Lord vampire. And all of this prophecy of the possible annihilation of vampires, the famous Tyranny of the Sun.
http://www.eurogamer.net/articles/2012-06-05-the-elder-scrolls-5-skyrim-dawnguard-previewSerana, it comes to play, is a very important part of the puzzle to find an Elder Scroll - the main quest - and to thwart the Tyranny of Sun prophecy that will wipe the vampires out.