The Stone Heart — Faith Erin Hicks

Kaidu and Rat have only just recovered from the assassination attempt on the General of All Blades when more chaos breaks loose in the Nameless City: deep conflicts within the Dao nation are making it impossible to find a political solution for the disputed territory of the City itself.

To complicate things further, Kaidu is fairly certain he's stumbled on a formula for the lost weapon of the mysterious founders of the City.... But sharing it with the Dao military would be a complete betrayal of his friendship with Rat. Can Kai find the right solution before the Dao find themselves at war?