30 yrs ago the Citadel of Weeping Pearls vanished, taking with it one of the Empress's daughters. Now the Empress needs her daughter's special talents to help fend off an invasion, but the only scientist to ever find traces of the Citadel has also vanished.
Reading the preview, I thought it was going to be a mystery, with the Empress's ex lover investigating the disappearances, but it didn't really fit any subgenre. I still enjoyed it though.