Meet the BBC 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. Also, at the end, I'm going to tell you what I think your story is about, based on your query. I know how hard it is to get your ideas across succinctly, and how easy it is for your author's brain to fill in the blanks and not see the gaping holes that the average reader may very well fall into.
Also, for my brave Saturday Slash volunteers I will gladly do follow-up slashes (each more kindly than the next) on your query if you post them on the Query Critique board over on AgentQuery Connect. You'll get advice from me, and also people who are smarter than me. If you do post on AQ, be sure to follow the guidelines and let me know you posted so that I can follow up!
And now for our next brave soul. For clarity, my comments are in purple.
Cara lives in quiet monotony, oblivious to the fact that her soul has been altered. Her ersatz father magically removed any hint of independence while she was still a baby making her eminently tractable. Now it’s only a matter of time before he trades her life for immortality. Holy vocabulary words, Batman! It's OK, I'm smart too, and I know what these words mean. I get that you're trying to impress the agent, but the thing is that the average reader might not know what all these words mean, which means the text might not be accessible. If your ms is sprinkled with words like this and the query is good representation of the book then I suppose this is alright, but if it's not then you need to rethink how you're introducing yourself here. The hook is buried under some pretty heavy verbiage, and if the agent thinks your plot might be too, you've got a date with the Delete button.
But when he removed her strong will from her magical essence I'd straight up use the word soul here, unless the magical essence is something else?, the girl’s inherent power warped the spell, creating an unseen second infant I know what you're actually saying, because I've read your query before, but what this says is that there's an invisible baby somewhere to house what she had lost. I love the phrasing here, and I think this first line is really strong. I'd try to incorporate this into your hook, as it's the crux of your story After he takes Cara to the Keep, a nursemaid elopes I know what the word actually means, but its modern connotation makes this read like she just ran off to marry an invisible baby with her new twin. Two infants, one soul, separated for twenty years with no knowledge of their true connection. Again, here's a great line. And again, it should be in your hook.
When her father's immortality ritual fizzles – neither killing Cara nor immortalizing him – he is forced to sacrifice regular men to bolster his flagging powers. Nice, well-phrased, concise. You need this language throughout. Captain Hunter arrives I'd rephrase to illustrate that he is a prisoner, arrives makes it sound voluntary at the Keep, slated for death, and shatters Cara's emotional isolation with a potent mix of attraction and guilt. YES good stuff here Moved by his plight, she plots their escape and meets her other half, I'd isolate this phrase with dashes instead of commas to make it stand out a warrior maiden named Falin, the other dash going here during their harrowing flight.
As they begin to piece together the terrible wrong done to them, her father discovers for himself strike for himself that Falin is the spell’s missing piece. The women must destroy him before he sacrifices them both. Great sinker - you've got the key struggle of the plot right here, but I'd reiterate that the point of the sacrifice is to successfully reenact the botched spell from years earlier.
I think what needs the most attention here is your hook, and most of my reasoning is spelled out above. I think your second para is unnecessary as it holds the main ideas you need in your hook. Rework that, cut and paste with elements of your opener and that second para to get together a tight, concise hook. The third para and your closer are spot on, with a little rewording I think you're looking pretty good!