User blog comment:G0LD3NF1RE/Skyrim Dawnguard DLC/@comment-4975320-20120501215945/@comment-5060344-20120502132610

Given that Sheogorath has already had his own DLC and Nocturnal was very prominent in the Thieves Guild questline I can't see it being them. Meridia is the most likely of the five I think.