Shadow Over Thunder Bluff

If you see an invasion of Dark Tendrils and Shadows of Uul then your account is likely involved in a phased event related to the unlocking of the Highmountain Tauren race on your account. You must finish the quest chain to unlock the Highmountain Tauren race in order for Thunder Bluff to return to normal.

Small word of warning:

I recently did this as a level 50 Frost Mage after the level squish (with non-heroic BfA dungeon gear) and it was an almighty pain in the arse. The Shadow of Uul mobs are immune to slow or freezing effects, and Bairne does a dismal job at holding aggro - combined with my squishiness it was not a recipe for a whole lot of fun.

In certain areas (like right outside of the tent where you start the quest) these mobs can suddenly rush in from nowhere in groups of 2-3 as they are scripted to chase NPC defenders, but they will quickly aggro to you. Because of the complete lack of crowd control options, you can end up getting eaten pretty fast. Even throwing up Ice Block didn't save me on a few occasions as they just started attacking me again the instant it finished, and it's one of the only times in recent memory that I have had to use mage food in between every fight because of the guaranteed damage these things inflict.

I died multiple times and each time I had to wait for the elevator near the graveyard at the base of Thunder Bluff, which sure as hell gets repetitive quickly.

The area where you need to kill Qy'telek (in the follow-up quest after this) is incredibly painful for this reason as well. If you pull a Shadow of Uul while fighting him as a class with no health regeneration options, prepare to die.

Is it only me or Thunderbluff keeps being phased in that quest even after it is completed... :/

With the Darkmoon Fair, it is even more ridiculous, with double version of each npc in the town ! one version is fighting while the other one is dancing ...