The Emperor's Knife (Tower and Knife Trilogy #1)

There is a cancer at the heart of the mighty Cerani Empire: a plague that attacks young and old, rich and poor alike. Geometric patterns spread across the skin, until you die in agony, or become a Carrier, doing the bidding of an evil intelligence, the Pattern Master. Anyone showing the tell-tale marks is put to death; that is Emperor Beyon's law...but now the pattern is running over the Emperor's own arms. His body servants have been executed, he ignores his wives, but he is doomed, for soon the pattern will reach his face. While Beyon's agents scour the land for a cure, Sarmin, the Emperor's only surviving brother, awaits his bride, Mesema, a windreader from the northern plains. Unused to the Imperial Court's stifling protocols and deadly intrigues, Mesema has no one to turn to but an aging imperial assassin, the Emperor's Knife.
As long-planned conspiracies boil over into open violence, the invincible Pattern Master appears from the deep desert. Only three people stand in his way: a lost prince, a world-weary killer, and a young girl from the steppes who once saw a path in a pattern -- a path that might save them all.