Blackhand's Command
This quest begins from the item "Blackhand's Command" which has a 100% droprate (and can be looted by the entire party) from the Scarshield Quartermaster, just outside the entrance to Blackrock Spire.
Objectives:
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.
Simply head through Upper Blackrock Spire (10-Man raid, requires the Seal of Ascension to enter. Only one player needs the Seal. UBRS can easily be 5-manned by level 70s) and defeat General Drakkisath (the last boss) and use the orb behind him.
You are now attuned for Blackwing Lair.