The premise of the book is divided into multiple story arcs spread across 600 pages of story. The primary characters are Penatanimir Benoist, who is the son of a murderous genocidal maniac who has inherited none of his father's darker qualities. His brother is similarly well-inclined. Penatanimir develops a romantic relationship with the human Brahanu and their brief romance results in a situation that threatens the stability of both realms.
The other characters involved in the story provide multiple perspectives on the conflict between the Nazil and their human subjects/enemies. Zeta the slave girl has suffered horrific rape and abuse at the hands of her previous masters, only to be dumped into the Benoist household which promptly treats her with such kindness she's confused as well as suspicious. Itai has loved Brahanu his entire life but discovers he's been replaced seemingly overnight with a child of his people's worst enemies on the way. Beilzen is obsessed with claiming his "rightful" position among the Nazil but his father has been so neglectful in teaching them about his people, he's completely blind to the fact he's nothing but a mongrel to them.
The Nazil are an inspired but all too realistic collection of villains. If I had to give them a set of inspirations I'd say they reminded me a bit of a cross between the Romans, Nazis, and the Confederacy. They're a brutally oppressive race obsessed with racial purity and clearly on the tail end of their empire's success. Virtually every one we meet is a monster with the term "ghosts" used as a racial slur for their people by their enemies (and seemingly not without reason).
Racism plays a role in the series as the central conflict is between the Nazil, humans, and the tiny community where the two sides live together in peace. While the Nazil are easily the worst of the factions, no one's hands are clean and prejudice is at every layer of society. The interracial relationships, both past and present, form the central narrative of the story. They render the characters as people who will have no place they can live in peace even as the larger conflicts threaten to consume everything.
A secondary theme of the book is the conflict between duty and love which George R.R. Martin discussed at length in his books. Unlike George R.R. Martin, attempting to follow duty isn't the "right" choice simply because of personal happiness. The simple act of not loving your partner, even if you're doing your duty by them, can and does have horrible consequences for our protagonists. They also cannot change how their perspectives have been altered by seeing the other side as worthy of respect.
As mentioned, this is a pretty lengthy tome and is two good books worth of excellent fantasy. The books can be a bit much in their sexual crimes and descriptions of the atrocities worked against slaves or characters but, sadly, are fairly true to history. The Nazil characters also tend to split along black and white lines with either a character being a slave-owning monstrous rapist or one who would gladly return one to their homeland. Either way, I was very entertained by this book and will check out the sequel.