The five families of Katar have ruled The Cites for thousands of years. Disobedience is punishable by complete draining of all of your ka, your life force, leading to true death.
When the Tarth, daemons of legend that the Katar banished five thousand years ago, return, the rebellion sees a chance to divide the Katar and break their rule forever. To accomplish this they need the help of the dying races, a thief, and one of the Katar.
Failure is not an option, but success could mean the Tarth destroy mankind in the wake of a weakened Katar.