The Elder Scrolls Online's first major content expansion, which will introduce the Craglorn adventure zone, will be released some time this week. Speaking to IGN, developer Zenimax said testing is coming to a close and that the expansion will rollout this Thursday or Friday.
As explained by Zenimax, the Craglorn update is geared towards higher-level characters. It's the first veteran-level, end-game zone designed specifically for groups of four players. There's even more special content designed for groups of 12 players. In addition to entirely new areas to explore, Craglorn introduces Trials, ESO's large group veteran PvE system that are designed "for the best of the best." You can learn more about Trials here.
The Craglorn update will be free to download, though ESO does require a $15/month subscription.