Even though the surprise has been somewhat spoiled by a Trophy leak, Bethesda's tease of the next Dishonored downloadable content sheds a little more light on it. A screenshot released by the company today hints at who you'll be playing as, and more importantly, when. Spoilers to follow.
The screenshot on Dishonored's Twitter shows a first-person perspective of one of Daud's assassins, apparently preparing to kidnap Lady Emily. That sets in motion the sequence of events that cause Corvo to get all murdery, so it seems likely this story will run parallel to the main game. The tweet itself promises "news tomorrow."
This revelation isn't terribly surprising, seeing as we'd already heard the DLC would involve Daud and was titled The Other Side of the Coin. But replaying such a pivotal moment from the other side should be interesting at the very least, and we'll know more soon.