The Beacon of Tyrhold.

The Beacon of Tyrhold is an immense titan-forged tower located in Thaldraszus on the Dragon Isles. A 10,000-year-old marvel of titan technology, Tyrhold was built by Keeper Tyr after the Dragon Aspects first settled on the Dragon Isles[1] and allowed him to do such.[2] When the dragonflights left the Isles in the wake of the Great Sundering, the titanic watchers used the Beacon to place the Isles in a deep slumber and hide it away from the rest of Azeroth, even from the dragonflights themselves.[3] In a time of need, the watchers would light the Beacon to remove the Dragon Isles' concealment and beckon the dragons back home.[4]

Once the time arrived to light the Beacon, however, most of the watchers had deteriorated. A lone surviving watcher awakened from his slumber, made his way to the top of the tower, and managed to reactivate it, but the resulting shockwave of energy sent him plummeting off the side of the tower. He was saved by the intervention of Alexstrasza herself, as the lighting of the tower allowed the dragonflights to return to the Isles en masse.[3]