Deathwing will randomly appear in a zone and burn it down. If your low or high level, you probably won't survive. And if a friend notifies you, you probably won't be there in time. The achievement is called [Stood in the Fire].
The sky will start to change color when he is about to show up so you will know. Obviously when that happens you will be toast....At least until Cata when you can hop on your flying mount and get high enough.