That is one stupid orc. It would appear as if you need to find this brand and gain the Mark of Drakkisath in order to access the Orb of Command.
The letter indicates that General Drakkisath guards the brand. Perhaps you should investigate.
I hold very little faith in your ability to prevent outsiders from accessing the master's lair. In the very likely event of your death, this orb has its own failsafe built in to prevent outsiders from teleporting directly into Blackwing.
Only those with the Mark of Drakkisath branded upon their hand may make use of this orb. Thankfully, Drakkisath is not nearly as incompetent as you, Quartermaster. He guards the brand himself!
-Warchief Rend Blackhand
P.S. Destroy this letter, idiot.
You don't even flinch as the smoldering brand is pressed into your flesh. You are so tough!
You now hold the Mark of Drakkisath. Have fun storming the castle.
Upon completion of this quest you will gain: