Book 6 of Ghosts. Spoilers for earlier ones.
After a murderous and mysterious attack on Caina, Halfdan has Caina, and the newly joined Corvalis and Claudia, on a mission to the city of the Masked Ones. They are known for their sorcery, and their seclusion, which makes it all the odder that they are offering to sell a marvelous weapon to the highest bidder. They pose as a jewel merchant, his daughters, and a guard, and reach it with the many ambassadors.
The plot involves rivers of molten steel, sisters, the promise the weapon is so terrible it will end war, question about maintenance, Kylon's noticing that Caina and Corvalis are in love, red iron and the merchant who sells it, and more.