The Dagger and the Flame by Catherine Doyle

In the dark underbelly of a beautiful city, two rival assassins are pitted against each other in a deadly game of revenge, where the most dangerous mistake of all is falling in love in this searing young adult fantasy.

In Fantome, a kingdom of cobbled streets, flickering lamplight, beautiful buildings, and secret catacombs, Shade-magic is a scarce and deadly commodity controlled by two enemy guilds: the Cloaks and the Daggers—the thieves and the assassins. On the night of her mother’s murder, eighteen-year-old Seraphine runs for her life. Seeking sanctuary with the Cloaks, Sera’s heart is set on revenge. But are her secret abilities a match for the dark-haired boy whose quicksilver eyes follow her around the city?
Nothing can prepare Sera for the moment she finally comes face-to-face with Ransom, heir to the Order of Daggers. And Ransom is shocked to discover that this unassuming farm girl wields a strange and blazing magic he has never seen before. As the Cloaks and the Daggers grapple for control of Fantome’s underworld, Sera and Ransom are consumed by the push and pull of their magic…and the deadly spark and terrible vengeance that keeps drawing them back together.