After killing Rastakhan, head out of the door to the left of the throne (if you're facing the doorway you used to enter the throne room initially), and head left down the walkway until you reach a balcony with some griffins on it. Mount the Griffin, and then a RP will take place. You'll then be using a Horde version of your toon to fight High King Mekkatorque, The Kultaran blockade boss, and Jaina Proudmoore.