The Stone Heart (The Nameless City #2) by Faith Erin Hicks

Kaidu and Rat are only just recovering from foiling the assassination attempt on the General of All Blades when conflict breaks loose again. Disputes and treachery are making it impossible for the Dao rulers to follow the diplomatic solution Kai's father has devised to give all peoples - not just the Dao - say in the future of the Nameless City.

Even more complicated, Rat may have the key to uncovering a vast weapon wielded by the mysterious founders of the City - a secret hidden long ago, and a secret Kai may be unable to keep. To reveal it would be to betray his friendship with Rat and his new home. But to betray the Dao may have dreadful consequences for everyone and everything Kai and Rat both love...