The Last Hours: Chaind of Gold by Cassandra Clare

Cordelia Carstairs is a Shadowhunter, a warrior trained since childhood to battle demons. When her father is accused of a terrible crime, she and her brother travel to Edwardian London in hopes of preventing the family's ruin. Cordelia's mother wants to marry her off, but Cordelia is determined to be a hero rather than a bride. Soon Cordelia encounters childhood friends James and Lucie Herondale and is drawn into their world of glittering ballrooms, secret assignations, and supernatural salons, where vampires and warlocks mingle with mermaids and magicians. All the while, she must hide her secret love for James, who is sworn to marry someone else.

But Cordelia's new life is blown apart when a shocking series of demon attacks devastate London. These monsters are nothing like those Shadowhunters have fought before - these demons walk in daylight, strike down the unwary with incurable poison, and seem impossible to kill. London is immediately quarantined. Trapped in the city, Cordelia and her friends discover that their own connection to an dark legacy has gifted them with incredible powers - and force a brutal choice that will reveal the true cruel price of being a hero.