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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.
Rowena Andalynn expected her homecoming to be joyous after a childhood spent as a diplomatic hostage to a neighboring country. This first sentence doesn't necessarily set this up as a fantasy. It's not until the second one and the inclusion of "court" that we get that feeling. She endured a treacherous court by honing her skills in violence and cunning. Survival forced her to ignore what she was born to be - a Wishkeeper, one who dispenses wishes granted by the Divine. Lots of good info here, but I'm not sure that it's a hook. I think you need to get the idea of being a diplomatic hostage and violence and cunning into one sentence for the hook.
Now she's allowed home awkward phrasing to assume her holy birthright. In the land of Boldenwhite, Wishkeepers channel Divine will to grant three wishes a year. Trained in wisdom, Wishkeepers are icons of grace for believers. Not sure there's anything in the underlined sentence that is necessary to the query.
But when her official return is marred by a disastrous first Wish that dooms not only her Order, but that of the government she serves, Rowena realizes that being a Wishkeeper does not mean unending blessings. And when Wishkeepers start dying, Rowena discovers that an enemy is rising...an enemy called Scian March. A product of Boldenwhite’s notorious prison, Scian has spent all her seventeen years planning revenge and revolution. The name actually sounds like an organization, so when I first saw it I didn't process it as a character.
In the battle over Boldenwhite’s future, Rowena battles on two fronts - against Scian’s bloodthirsty allies - and with everything she thought she believed. which was? The calling that defines her may become the trap used to kill her world, how so? if Rowena doesn’t win The Wishkeeper’s War. is it on her alone?
For the most part this is a good job of getting plot, world building, and conflict into a tight space - not easy with high fantasy. However, some elemental aspects are missing. Rowena battling against "everything she thought she believed" is, I think, a callback to her realizing that "being a Wishkeeper does not mean unending blessings," but it's not explicitly clear. If Rowena has had no training in her gift because of her upbringing and is therefore somewhat naive about it, that might be something to clarify.
Also, clarification on what precisely is at stake would be good - how are the Wishkeepers gifts being used against them? Does Rowena have any allies herself? You mention that Scian does, but it seems to be on Rowena alone to win the war, which seems a bit odd given that she hasn't lived in Boldenwhite for most of her life. Some more details about Scian would be good too - why was she in prison in the first place? Does she have some dark power? Clarify some key points, pep up that hook sentence, and I think you're ready to take this query out.