The Hour of Twilight is the fulfillment of events long planned by the Old Gods.  An ancient prophecy speaks of the Hour of Twilight, the final days of the world.

If the Old Gods set their plan in motion, this chain of events would reshape the world in their dark image.  Then, heroes unknowingly traveled to Ahn’Qiraj, and broke the ancient seals.  C’thun stirred in the deep, and N’zoth, long sleeping, began to stir.

Amidst the horrors of the Cataclysm, ancient machinations materialize as Deathwing rises over Azeroth, wreathing the world in flame.

If the Hour of Twilight is not overcome, every life on Azeroth will be dead and rotting, as the Twilight Dragonflight rises to claim a new birthright.  In Ysera‘s vision of the Hour of Twilight, all five Aspects–including Deathwing–lay dead at Wyrmrest Temple, killed by their own powers.