Deathwing will soon be here, attacking from above, bringing down terror and death onto the unsuspecting citizens of Azeroth.
At random, Deathwing will attack an entire zone and kill everything -- players, NPCs, mobs, creatures and critters. For your death, you get the feat of strength Stands in Fire. We don't know when exactly this will become active on the live realms; however, we are able to confirm it's active on the beta realms.
The full blue announcement as follows:
One of Azeroth's oldest adversaries will soon tear through the Elemental Plane to reforge the world in flame, but this foe won't be conducting his reign of terror from the confines of a castle or lair. Deathwing is a destructive force the likes of which adventurers have yet to see in World of Warcraft, and his appetite for devastation can only be satisfied by the shattering of the world. Following his explosive escape from Deepholm, Deathwing will cast a deadly shadow over the people of Azeroth as he wreaks indiscriminate havoc across the land.
Once free to terrorize the world, Deathwing will randomly choose territories in Azeroth to attack each day until his ruinous reign is brought to an end. A blackened sky will be the only warning before every living creature caught in his approach is consumed by his terrible fire. The unfortunate victims of his malice will receive a rare Feat of Strength... as well as a repair bill and corpse run.
When Deathwing returns, you'd best keep your eyes on the skies.