This item is the axe Broxigar used is the War of the Ancients trilogy of novels.
It was crafted for him by Cenarius, after his own axe shattered. He used this axe to attack Sargeras. Though this did not signifcantly damage Sargeras, Broxigar succeeded in creating a focus point for the spell work to stop Sargeras, but at the cost of his own life.
Krasus returned to current time after the events, and gave Thrall the axe, telling him of Broxigar's bravery and death.
We don't know what became of the axe after this time, or have many clues as to where it may reside. Many speculate that it is now in the hands of Broxigar's brother, Saurfang, High Overlord of the Horde.
No one has any idea how this item may be implemented into the game in the future regarding lore.