The Stormcrow mount was the hint, it's so Blizzard to do this.
And this is it:
"When the sky was torn from the sea, Storm Crow appeared from within the thundering clouds to grant mankind dominion over the living flame." —Legends of the Forgotten World
They are very clever, they used Diablo 3 to give us the clue.
So what does it mean that the "sky was torn from the sea"? in sense of Warcraft and the next expansion.
And "grants mankind dominion over the living flame"?
And more importantly, what is the Forgotten World?
These quotes are all originally from Warcraft, not Diablo, but are used within Diablo 3 on that item, as you see.