The setting for this new DLC appears to be Solstheim, the island that was last seen in The Elder Scrolls 3: Morrowind as post-release expansion pack. The story appears to focus on an evil Dragon Priest, the former ruler of Solstheim who preferred feasting on dragons to serving them. In fact, it seems he was the FIRST Dragonborn.
The trailer is also overflowing with looks at a variety of new enemy types and what appears to be a new alternate form (a la werewolf/Vampire Lord). The Dragonborn DLC will be available on Xbox Live starting December 4, 2012 for 1,600 MS Points.