If I recall correctly, the folks at ZeniMax Online were open about their intentions to add a Dark Brotherhood storyline to The Elder Scrolls Online before the game's release, so it's not exactly a surprise to learn that The Dark Brotherhood is going to be the next DLC for the title..
Before we dive into the details, with the help of the game's official website, here's the teaser trailer:
From what Bethesda disclosed, the DLC will take place in the westernmost region of the old Empire, the Gold Coast, and feature a completely new storyline, repeatable quest content involving Contracts and Sacraments, a new passive skill line, and unique items. A patch will also be making a number of significant changes, including the addition of poison-making, the removal of Veteran Ranks, improved grouping tools and more.
Here's the full changelog, for those of you who want all the details: