We all know the first line of a query is your "hook." I call the last line the "sinker." You want it to punch them in the face, in a nice, friendly kind of way that makes them unable to forget you after having read the 300 other queries in their inbox.
If you're looking for query advice, but are slightly intimidated by my claws, blade, or just my rolling googly-eyes, check out the query critique boards over at AgentQueryConnect. This is where I got my start, with advice from people smarter than me. Don't be afraid to ask for help with the most critical first step of your writing journey - the query. My comments appear in green.
In a world where magic is banned, Haden has a unique ability that makes him one of the best spies in Syva. Hmm... okay but what is the connection of his ability to magic? Is it a magic ability? The reader can kind of assume that, I suppose, but technically the way the sentence is structured it doesn't say that. Too bad that isn't the life he chose for himself.
Haden Gensmith counts the days until he can finally pay off his debt, allowing him to be free to leave the service of The Rat—Syva's very own spymaster—to travel the world. When he receives his final mark, he's surprised to find his last assignment will be to gather information for the assassins about Princess Vena, cousin to the king of Syva. However, Vena is not the snobby and privileged girl he expected. Instead, she's kind and understanding and just as trapped in her life as he is. But she has a deadly secret, and when Haden learns it, his loyalty is tested as he refuses to pass the information on to the assassins. After all, he'd never expected to fall for the girl he was supposed to help kill. This is good - however I still don't know how magic comes into the plot?
When the spymaster learns of Haden's betrayal and tries to have him killed, Vena reveals an even darker secret and a new set of skills, saving his life. Together, they set out on a journey to defend both of their lives and discover the truth of why everyone wants them dead.
CLOAK & DAGGER is a YA fantasy with series potential, complete at 65,000 words. I am an avid reader, book blogger, and a member of AWP.
Okay cool, this is actually pretty good. You've done a good job of showing us what the setup is, but the plot itself is pretty generic -- character falls for person they are supposed to be the undoing of, right when they're about to realize their own personal goals and have to decide what's more important.
The big question here is - what makes your story different from every other story that fits into the trope I mention above? You mention magic once, and also Vena's darker secret and special skills, but we don't know what they are. These are the elements that make your story unique from every other novel that fits into this plot - get them into your query.