From the description, I thought this book might be all sex. But it's not. It's darkly erotic, but sex doesn't overpower the story.
So this story is dark fantasy romance/erotica. Ava is a Thief. She devours magic, mostly through cannibalism. She keeps her hunger under control, though lately it has been getting harder and harder. She's in love with the mage, Reist. He found her living on the street and took her in at the Institute and now she works as his spy. But lately she has had to watch him fall in love with another mage and it's tearing her apart. In desperation she goes to Heyerdar. He is an Elemental, a person with old magic. Heyerdar has an interest in breaking them up too; the mage sleeping with Reist left Heyerdar for Reist. Ana and Heyerdar can use their magic together to push the two apart.
That's not the only reason these two come together, though. Thieves, like Ana, have gotten past the wards and into the city. They've already murdered a family. Ana is assigned to work with Heyerdar, as he's the Captain of the Imperial Guard, to find the killers.
Heyerdar can be quite crude. He's not always very nice to Ana, but it's obvious he's pushing Ana to see how she is being used by others and really, to be her own person. I liked these two together a lot, and couldn't wait for Ana to realize her own true feelings.
Between the sex magic and cannibalism, it's a dark story. It's very well written and the characters are fascinating. I wish there had been a bit more world building. And I found Ana's back story a little confusing. But other than that, I enjoyed this story very much.