When doing the legion quest Dark Omens at the Stair of Destiny a portal should be available afterwards by the Dark Portal to take you back to Stormwind. However, a portal does not appear where indicated, likely because the area was phased for the quest and the portal was not included in that phase.
I had to mount up and leave the area until I reached The Path of Glory, then return to where the portal should be and it was now there. That seems to reset the phasing. It makes sense to add or modify the portal so this is unnecessary.