Nazafareen's sister Ashraf was killed by the Druj (Undead things with iron swords and shadows whose touch meant death) when Nazafareen was twelve and Ashraf was seven. Now, all she lives for is revenge.
When the authorities-that-be discover she has the power to link with a daeva she willingly agrees to do so if this means that together she and the daeva will be a match for the Druj and able to hunt and destroy them. At first, she distrusts the daeva, whose name is Darius, thinking of him only as another kind of Druj but tamed and under her control – litle more than a sentient weapon. But living together, linked like that, she and Darius find themselves growing too close for her comfort in other ways.
This is an alternative version of ancient Persia and features a form of the dualistic Zoroastrian religion, in which two Gods fight an endless war, and people have to choose which side they are on, the Good or the Evil. (I have always found this form of dualism much more philosophically tenable than strict monotheism.) It also features both the prophet Zoroaster, the founder of this religion, and Alexander the Great, though here in this book they remain in the background; in Book 2, Blood of the Prophet, which I have already started reading, they both move into the foreground.
Extremely well written and highly recommended.