The Emperor's word is supreme, but the Emperor rarely speaks. As such, it is the Dark Council that rules and administers the affairs of the Empire while the Emperor is silent, determining the fate of worlds and trillions of sentient beings. Each Dark Council member takes charge of one of the twelve pyramids of power within the Sith Empire, and may act as he or she sees fit.
In practice, joining the Dark Council is just the beginning of a Sith's true rise to prominence. Within the Dark Council are alliances between members designed to keep other members weak or ensure a majority in issues that must be decided by the council as a whole. While in theory there is no head of the council, in practice, every member is struggling to amass enough personal power to control the others. Those who have been on the council for years or decades have deep roots.
While the Dark Council rarely meets in full, a canny Dark Council member makes it a point to be aware of every meeting, and to always have an ally in the chamber.