The Elder Scrolls Online Harrowstorm DLC is the first installment of content for the year-long story, The Dark Heart of Skyrim. Specifically, the Harrowstorm DLC will be featuring two new dungeons; The Unhallowed Grave and Icereach.
Unveiled early January, Elder Scrolls Online announced the new year of content. This content will be dropped to the ESO community in 4 installments, one for each quarter of 2020. As tradition, each content release will be separate purchases. Unless you have ESO Plus, in which case, only the Chapter will require purchasing.