Nether-Stalker Mah'duun wants you to kill 4 Rift Lords. Return to him in Shattrath's Lower City once they all lie dead in order to collect the bounty.
Mah'duun knows where you must go. Mah'duun knows that you must kill rift lords. But, Mah'duun does not know exactly when.
I know that does not make sense, but the voice on the winds was not more specific.
Travel to the Black Morass through the Caverns of Time and the way shall be made clear.