Before the release an introductory quest showed up in-game telling a story of how people were attacked by their own shadows due to some daedric influence. With the release of the DLC Divayth Fyr shows up in your alliance capital and asks you for help in getting inside the Clockwork City, Sotha Sil's realm which is hidden in a different dimension. Getting there you realise that the shadow problem is there too and that there's a plot against Sotha Sil that you have to uncover and stop.
I really enjoyed the look of the Clockwork City and the two world bosses were really cool. However the delves were not really to my taste. All the quests were really cool, though I'm not a fan of the Blackfeather Court daily quest.
What was amazing was the new trial: Asylum Sanctorium. It's a mini-trial so it's not much bigger than an ordinary dungeon, with the exception that you need twelve people rather than four to complete it. I've only done it a few times, but we did really well those first few times. The last boss was of course the hardest, but we didn't wipe as many times as we did with for example Halls of Fabrication or Maw of Lorkhaj. Actually after just a few tries we didn't wipe at all, though many people died from time to time we always had great teamwork and ressed people as soon as possible so that we always had a team.