Hall of Binding is an area of Upper Blackrock Spire. It is accessed through the sealed door in the Dragonspire Hall by defeating all the orcs in that area. This is where several Blackrock channelers are keeping Pyroguard Emberseer, minion of Ragnaros, bound through force of magic. This magic can be disrupted, which will cause the orcs to attack in a frenzied panic, and Emberseer will break free of his elemental prison when all the channelers are dead.
The Hall of Binding leads on to the infamous Rookery, but Emberseer has to be defeated for the door to open.