Dragon assassin

Carmen is a student at Red Assassin School. She's an expert at bladed weapons and poisons; and she's desperate to finish at the top of the class, ahead of her twin brother. The students have been trained to hunt using giant black swans, but Carmen has discovered a dragon. All she has to do is get on his back. One problem: he's killed everyone who gets near him. Then the Emperor declares war on assassins. And there might be a traitor among them. C...