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.
Blood Reign is a YA Fantasy novel complete at 78,000 words, that will appeal to fans of Kristin Cashore’s Graceling and Tamora Pierce’s Song of the Lioness series. Great - sounds like you know your genre and comp titles, but I personally put what I call "the vitals" at the end of a query letter. It's not a hard and fast rule, but I think it's much more interesting to put your hook out there front and center. Also, after having finished the query I'll add that if this is an Alice retelling, this is the place to mention that.
Seventeen-year-old Alice never considered herself the suicidal type. Oh, interesting - definitely raises the question of what changed her mind on that count. That is until she finds herself trapped between the men who killed her mother, you don't need the comma and a five-hundred-foot drop. Rather than face the killers’ dark plans for her, Alice jumps. But instead of death, slightly awkward phrasing here since you continue with a verb. I'd rephrase with "instead of dying" Alice wakes up in a blood-soaked battlefield in slightly clunky sentence here, plus echo with "in" an unfamiliar world, where men in armour are slaughtering peasants. The slaughter you mention here implies the blood and leaves you able to pare down this sentence. Plus (picky moment) if the peasants are being slaughtered then it's not technically a battlefield, but a killing field. Terrified, she flees and encounters a seer, who reveals the only way to return to her world is to seek a witch. Interesting enough, but I'm definitely confused about how her mother's death / murder / killers fit into this plot.
Disguised as a man why?, Alice must survive the war between the queen and rebels who is the queen and why are there rebels?, and the flesh-eating monsters stalking Wonderland So we are actually in an Alice-In-Wonderland retelling? Why are there flesh eating monsters?. Her growing lust for vengeance makes her determined to find her way home. Why? To avenge her mother's death? As Wonderland falls into chaos, Alice discovers slaying monsters might have its price--being hailed as a hero--or becoming a monster herself.
This definitely all needs to be tied together. The mother's death feels like the impetus - it leads to her suicide - but then it's dropped entirely and only revisited as a mention later on through implication. There is a mention of a witch who needs to be found but then she disappears. The queen, rebels, and dead peasants are all in evidence but their relevance to the plot is not clear, and neither is the presence of flesh-eating monsters.
You also have all the elements of this being an Alice in Wonderland (with zombies?) re-telling, but you never actually say so, which you probably should if that is, in fact, the case. Her newfound attraction for killing is a great plot point, but it's getting lost in the cast of characters who don't seem to all fit together cohesively. I'm also very unclear how Alice became a slayer of monsters in the first place, or why she is disguised as a man.