http://www.pcgamer.com/the-elder-scrolls-online-clockwork-city-release-date-announced/

The Elder Scrolls Online is heading to Sotha Sil's Clockwork City for a ten-hour expansion, due out on October 23. The update is included as part of the ESO Plus membership, and otherwise costs 2000 crowns in the in-game Crown store. That's roughly $20 / £15 worth of crowns once you've converted the amount from real money.

According to the TESO site, the pack adds a standalone story "similar in size to the main quest lines found in the Dark Brotherhood or Thieves Guild DLC". The story focuses on a conflict between Daedric Princes that threaten to pull you into the realm of Oblivion at points during the adventure. The update seems to otherwise be largely set in the mechanical clockwork city, built around a hub area called the brass fortress.