The thing to understand is that the Grand Council is not there to rule The Empire. It is there to not rule The Empire.
- The History and Governance of The Empire, by Aliss Barthenon
The Grand Council was a political body created by the first Emperor, Eramus the Conqueror. Each of the High Houses held a seat on the Council, and it existed to hear the grievances of noblemen, to decide issues of succession and to rule in the absence of the Emperor, as well as advising the Emperor on matters of state.
The Grand Council fragmented and dissolved seven years ago, an event that marked the beginning of the War of the Throne.