The Knife of Dunwall DLC will star Daud, the "legendary assassin that puts the game's plot into motion." Players will have a chance to explore new parts of the city of Dunwall, including a "steaming factory filled with colossal whale carcasses." (Sweet!) There will also be new powers, like "Summon Assassin," as well as new gadgets and weapons, like "Chokedust" and a "concealed Wristbow." (I think those are all pretty self-explanatory.)
A new single-player adventure certainly sounds more fun than the previous DLC, Dunwall City Trials, which just had a bunch of challenge maps with no overarching storyline. It looks like Arkane Studios and Bethesda have more single-player goodness up their sleeve, too. The announcement teases The Brigmore Witches, an "upcoming final add-on" that will complete this new side-story.
In the meantime, you'll be able to grab Knife of Dunwall on April 16 on Steam, Xbox Live, and the PlayStation Network. The DLC will cost $9.99 or 800 Microsoft points.