User blog comment:Blood Reaper/Skyrim Dawnguard Trailer/@comment-2.120.147.115-20120601163612/@comment-4118895-20120601191044

I'm guessing The Falmer is holding the other Elder Scroll. I'm guessing if you join the Dawnguards you help the civilized falmer AKA the last of the Snow elves fight against the Vampires and have a direct assault on the vampire's realm. Its vice versa if your vampire. This is just a guess