Just not for WoW.
You may have heard of Destiny, the much-anticipated upcoming MMO from Bungie -- the studio famous for its creation of the Halo franchise. It's to Bungie that The Dark Below belongs, and it's the title of Destiny's first expansion. You see, those interested players who purchase one of the two Destiny collector's edition releases: Destiny: Ghost Edition or Destiny: Limited Edition will get special access to the Destiny Expansion Pass, which includes Expansion 1: The Dark Below, and Expansion 2: House of Wolves.
So there you have it, everyone. The Dark Below is alive and well, albeit in an altogether different virtual universe, owned and developed by a different studio in a different state. Thus ends forever the mystery of The Dark Below. May it rest in peace, and live up to its, uh, Destiny.