The novel is centered around the city of Sorlost, which is something akin to Medieval Persia combined with China and ancient Carthage. It is the richest city in the world and one of the most powerful empires in the world but tradition has caused the city to stagnate. Orhan Emmereth is a nobleman who sees the problems they face and decides to overthrow the current manchild (possibly simple-minded) Emperor in order to establish a new dynasty. Unfortunately, this carries grave risks as the Empire's punishment for such coups is the extermination of a noble's families, servants, slaves, and more until nothing remains.
Elsewhere, Thalia, the Empire's high priestess presides over daily human sacrifices from adults to children. Not so much disgusted with these practices but bored, Thalia dreams of running away and finding a handsome prince to live a life of something other than service. She's lucky such a person exists in Marith, one of the mercenaries hired by Orhan who just happens to be a dragon-slaying descendant of gods. He's also a drug addict, murderer, and quite possibly insane.
Our heroes, ladies and gentleman.
Grimdark is a label which is casually thrown around by reviewers and readers nowadays. A lot of times, it means different things to different people and that actually ruins any value it might has as a descriptor. In the case of TCOBK, it is a description of an evil and corrupt society with a number of very bad people trying to survive in it. Every single main character is a mixture of good and bad but their actions would have them be villains in other fantasy works. Yet, we, the audience are able to cheer them for their attempts to carve out new lives for themselves in truly awful circumstance. We also might recoil against the actions they're willing to take (and so do they).
Sorlost is an awful but believable sort of place where human sacrifice is a valued cultural tradition. For example: the biggest problem people have with the rites is the little girl officiating them (after Thalia's escape) is prone to crying as well as wetting herself. It's an empire built on slavery and violence. You want to see it torn down stone by stone even as the characters in the story desperately want to save it. There's no "good guys" to contrast Sorlost to, either, so it's fall would just get a lot of people killed in the process. Yet, there's a very real question if Sorlost can be saved because it is s a society that is sick and apathetic at heart.
There's a lot of great, memorable moments throughout this book. Scenes which stick in your head as great triumphs or bitter defeats for our (anti)heroes. These are contrasted to the more personal human moments where we see the main characters reflect on how they've become what they've become. There's a great mixture of action and drama throughout the story and it benefits the enjoyment factor immensely. Actions have consequences and we see the results followed up four or five chapters later, making it feel more real and organic.
Marith is probably my favorite character even though I absolutely despise him as a person. An entitled brilliant military commander with charisma to spare, he's also a thorough wreck as a human being and has no limits to the amount of things he's willing to do in order to achieve his ends. It's as if he's Joffrey with Daenery's plot immunity. Despite that, I believed Thalia was the one person in the world he cared about even as she probably made a mistake not running as far from him as humanly possible.
In short, this is a book full of reprehensible but interesting people. There's lots of action, intrigue, character development, and presentation without justification. Anna Smith Spark never tries to make her heroes likable and that, ironically, means their actions feel like something we can judge or justify for ourselves. It's a dark-dark book and not something I would recommend for people who like fluffy reading but I found it immensely entertaining. I think I might have liked it better if the ending had gone where I thought it was going (and the book narrowly concludes with) in a bedroom scene with Thalia and Marith but such was not to be.