The Spell of the Black Dagger (Ethshar series Book 6)

The author writes: "Long before, I'd come up with the idea for the Black Dagger itself--it's a mildly clever variation on your standard parasitic sword, like Elric's Stormbringer or one of Steven Brust's Morganti daggers, and I've no idea how I came up with it, but I'd included a very brief mention of it in an earlier proposal, and Lester had liked it. So that was my starting point, and that gave me the entire opening of the story, showing how Tabaea acquired the dagger. The next step ... well, I'd noticed that all the previous Ethshar novels involved a lot of travel, so I decided I wanted to keep this one entirely in a single city. I chose Ethshar of the Sands because I'd used it the least of the three Ethshars."