Today is the first day of the new living story, the Nightmare Within. Unlike recent previous living stories, you can finish the meta (10 achievements from the Nightmare Within and/or daily living story achievements) on the first day to obtain the meta reward, Gas Mask Skin. However, these achievements are extremely difficult to complete if you are on a low population server. My advice is to guest on one of the high population servers like Tarnished Coast or Blackgate and try to get into overflow. Most of the achievements are pretty self-explanatory. The nine nightmare chambers are located on the map below.
Be warned that some of the chambers are located at the end of a crossroads with the other end being the way forward. It is advised to follow a commander who is giving a tour of the tower to visit each chamber in order to not get lost. I managed to solo the final story instance on my guardian but it is definitely easier and faster to do it with a party. Clearing the final story instance at the top of the tower unlocks an additional two achievements, one of which awards the Mini Toxic Nimross. So cute! ❤