Meet my Hatchet of Death (or, some other colorful description RC Lewis and I come up with at any given moment). This is how I edit myself, it is how I edit others. If you think you want to play with me and my hatchet, shoot us an email.
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.
When Princess Nel’s beloved Uncle Invidus vanished ten years ago, all she wanted was his safe return. Now that he’s come back and killed her parents, Nel wants nothing more than revenge. Interesting. Decent hook. I definitely want to know what's going on here.
With hopes of becoming an all powerful wizard, Invidus uses black magic to murder Nel’s parents, seize the Western throne, and awaken a deadly demon that will drain magic from every living creature and restore it within Invidus. When Nel sees how powerful Invidus is becoming she decides that she must stop the demon at all costs.
To do so, Nel must first uncover the demon’s tragic origin and she soon discovers that she is one of seven souls capable of destroying it. Despite her fears, Nel travels to an island of witchdoctors who agree to help Nel piece together the past by awakening Invidus’ deceased victims. Once Nel learns that the demon can only be destroyed through human sacrifice, she must decide if becoming a killer is worth saving her kingdom. Interesting. Yes, this is all well-written and very concise, but at the same time there's not a lot of emotional punch here. How does Nel feel, past the hook? Is she alone in this quest? Is there a love interest? You've done a great job of being concise about the main thrust of the story, and the protagonist's predicament is clear, but I feel like this is a very factual presentation that might need some more blood in it. For example - she awakens the deceased victims? Awesome!! Um... that includes her parents, right? Woah. Feels like a decent subplot there - is that the case? If so, maybe talk about it a little.
In short, there's really nothing *wrong* here. In fact, it's awesomely concise... maybe just a tad too drained of voice and blood. Give us a little more. Also, it might be good to state Nel's age at some point, we can assume YA but we all know what happens when we assume.