Steal the Defias Tower Key, break into the Defias Tower and uncover the contents of the Duskwood Chest. Take whatever information you find back to Mathias Shaw in Stormwind. Read Kearnen's Journal to gain insight about the tower.
I've been staking out this tower for weeks, [player]. Recently a major shipment came through from Duskwood, and along with it came those guards.
What are they? Your guess is as good as mine. I can tell you one thing - they aren't human.
Anyhow, the shipment had a chest - which is what I assume those abominations are guarding. The zombie with the spikes holds a key we'll need.
Get that key, get in the tower, and uncover whatever is in that chest. Take your findings back to Mathias.