They've managed, intentionally or unintentionally, to pull some damn good fridge brilliance before the game has even come out. Out of all the guilds announced, there is information on both the Fighter's Guild, and the Mage's Guild. But the Dark Brotherhood and the Thieves Guild strangely have no information available at this time. Let me repeat that, the Thieves Guild and the Dark Brotherhood have no information at this time. The two most secret guilds in the Elder Scrolls universe.
The two most secret guilds that we have been able to join. Beyond organizations so secret we aren't aware of them, the Dark Brotherhood honestly isn't that much more secret than the Necromancers.
In the course of five games, Molag Bal, the Daedric Prince of Domination, has shown himself to be quite possibly the most evil thing in the franchise. And do you want to know something else? He's only ever appeared in sidequests! That right, folks, he's been a distraction before, but with Online, the King of Rape now has his undivided malevolence set upon you. If you thought the atrocities he has committed as a footnote were bad, just how bad do you think he'll be when you have his full attention?
On the other hand, as Fridge Brilliance, this justifies the MMO itself. What do you do when you're facing the local embodiment of evil? You send in The Chosen Many.
Even moreso if you become a vampire, now that Zenimax has basically confirmed that you can become one. Since Molag Bal is the patron Daedic Prince of vampires as well as the source of their power... that Deal with the Devil just got worse.