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.
Miles Singer didn’t deserve to survive the war, but at least he’s alive to help. It's not a bad hook, but there are too many unknowns -- what war? Help who? Nightmares of evil beings so as of right now I don't know what genre we're in here - magical realism historical? fantasy? SF? haunt the patients of the veteran’s hospital where Miles works as a psychiatrist, but the ex-surgeon psychiatrist and surgeon? will root out the cause of their trouble even if he has to risk using magic to do it.
He’s off shift when Tristan Hunter leaps out of a carriage definitely need to know what the genre is here - more hints earlier with a dying witch in his arms. Nick Elliot who is Nick? They dying witch? begs Miles to save “the others.” When Nick’s body is stolen before Miles has the chance to prove murder, he agrees to help the handsome foreigner pursue Nick’s killer.
Tristan is more than foreign - he’s a Fair One, a capricious descendant of the messengers of the gods, on a mission to learn why magic no longer flows from Aeland is this the name of the setting? to his true home. Nick’s death may not be connected, but it’s the best lead he’s got.
When Miles’s sister learns he faked his death we didn't even know Miles faked his death - why would he do that?, she drags him back into the affairs of his family and the secret magical group that serves Aeland’s Queen. But Grace who is Grace? His sister? must prove she’s strong enough to lead the Invisibles Who are the Invisibles? Why are we just now hearing about them / their name at the end of the query? by enslaving the brother why would she have to enslave him? who escaped his fate fifteen years ago.
You've got a lot of names and allusions in this query that muddy the waters quite a bit, making it very difficult to see how all these plot lines connect to form a cohesive whole. How do Tristan, Nick, Miles, and Grace's stories all intertwine, other than through degrees of separation? I think you need to identify the main thrust of the story, and which character(s) best define that and focus the query on them, their goals and motivations.