The risen empire - Scott Westerfeld

The undead Emperor has ruled the Eighty Worlds for sixteen hundred years. His is the power to grant immortality to those he deems worthy, creating an elite class known as the Risen. Along with his sister, the eternally young Child Empress, his power within the empire has been absolute. Until now.

The empire's great enemies, the Rix, hold the Child Empress hostage. Charged with her rescue is Captain Laurent Zai. But when Imperial politics are involved the stakes are unimaginably high, and Zai may yet find the Rix the least of his problems. On the homeworld, Zai's lover, Senator Nara Oxham, newly appointed to the Emperor's War Council, must prosecute the war with the Rix while holding the inhuman impulses of the Risen councillors in check. If she fails at either task, millions will die.

And at the centre of everything is the Emperor's great lie: a revelation so shattering that he is willing to sanction the death of entire worlds to keep it secret ...