Make your way to Legion Hold, locate the Legion Communication Device, and learn about the Legion's plans while in disguise. Return to Blood Guard Gulmok at Shadowmoon Village when you've completed your mission.
The fortress of Legion Hold is your target. You'll have no reinforcements, so a frontal attack is out of the question.
What little information we have shows that the Legion stocks and prepares its infernals at the hold. With the help of a disguise I'll give you, you must sneak into Legion Hold and discover their plans.
Your best hope is to intercept or overhear communication between their commanders.