Just over half of the inhabited star systems in Shayla's time are ruled by one of six Grand Families.
After a period of relatively peaceful interstellar expansion, civilization collapsed in a thousand years of bitter conflict as fledgling empires grew impatient with the slow process of terraforming. Humanity as we know it nearly ended there, but a few centers retained both industrial capacity and technical knowledge to build starships.
The rulers of these few centers founded the Grand Families. With their unmatchable technology and wealth, they subdued nearby systems and have kept their grip on their territories ever since.
The Family Skamensis was always the most powerful, and holds a commanding central position in human space. Successive Skamensis rulers styled themselves "Emperor", with greater or lesser success. At the time of Ghosts of Innocence, the Skamensis rule is strong, with the other Families signing treaties of allegiance, so the current Emperor can proclaim himself Emperor of all humanity with some justification.
Of course, that doesn't stop the Families plotting constantly against the Emperor, and against each other. This is what gives Shayla the opportunity to get close enough to the Emperor to destroy him.