Can't remember the title

Hey all, I'm trying to find a book I read some time ago, following a boy who was found by a city guard captain, chained in a hidden compartment at the back of a cargo boat. The boy showed extraordinary abilities, much like those of a vampire, with similar weaknesses. He was entered into training as an assassin, but was later removed from his position due to failing a test, after being sabotaged by his rival apprentice. I can't for the life of me remember anyone's names from the story, but I remember the boy it follows, being told to flay a girl as punishment for her crimes, he fights a prince on a rooftop, whom he was supposed to assassinate.
This sounds like The Fallen Blade by Jon Courtenay Grimwood.