Travel to Blackrock Spire and slay Warchief Rend Blackhand. Take his head and return to Orgrimmar.
Rend dares make such grand claims because of the protection he is afforded by the black flight.
You, [player], will find a way to pass through the Halls of Ascension. You will then find 'Warchief' Rend Blackhand and you will destroy him - FOR THE HORDE!
<Thrall slams a fist down upon his throne.>
The next time you return to my chambers, you will hold his head high in triumph and then you shall present it to your Warchief. Do this and be honored as a hero of the Horde.