User blog comment:Blood Reaper/Skyrim Dawnguard Trailer/@comment-4772788-20120604005235

I think I know who the dawnguard are, they are worshippers of the daeric prince Meridia. Meridia is a daedric prince whom does not have much known about her other then that she hates the undead and necromancers. This makes her a very likely candidate for the patron of the dawnguard, especially since she tells you "A new dawn is approaching and you will be its herald" when you do her quest, also the weapon you get is the "dawnbreaker" due to these you can assume that the dawnguard are meridias worshipers against molag bals. This also leads me to believe that this dlc will teach us many things about Meridia. The only other groups who could be associated with dawn would be Azuras worshipers or Stendars or Arkay. The problem with Azura is she has no qualm with the necromancers, only the Dwemer and other worshipers of the daedric prince of knowledge. Arkay and Stendars worshippers have several outposts in skyrim. The problem with Arkay is his worshipers tend to the halls of the dead throughout skyrim so it is unlikely that they would take part in the fighting of vampires, this leaves Stendar and Meridia, the problem with Stendar is they have nothing that would get them the name "dawnguard", nor are they dressed the same way (Stendars worshippers wear robes of a priest, not the armor worn by the dawnguard).

TLDR: This DLC is of Meridia + Worshippers vs Molag Bal + Vampiric worshippers