Blackhand's Command

Blackhand's Command is a letter from Rend Blackhand to the Scarshield Quartermaster guarding the entrance to Blackrock Spire. The letter says that only people branded with the Mark of Drakkisath can enter Blackwing Lair.

This letter starts the quest.

Source
This item drops from the Scarshield Quartermaster in Lower Blackrock Spire. He can be found across from the entrance to Upper Blackrock Spire.