There's a titan device deep in a place called the Halls of Origination. It's meant to be an emergency reset switch for all life on Azeroth. If activated, it will destroy every living thing in the world!
Now, it's only meant to be used under certain extreme conditions, but if Deathwing is able to corrupt the titan watchers, he could use the contents of the coffer to trigger the device.
Go to the Obelisk of the Moon, <name>. Slow them down!
While working on Missing Pieces at the same time, once both quests are finished, a cutscene will be triggered showing one of the Schnottz workers discovering the body of one of his comrades. He raises the camp alarm, and players see Myzerian take on his black dragon form.