Firstly, how close are you to finishing the manuscript, or are you currently simply revising? If you try to put together a query too soon, you could find yourself omitting scenes that you add and referencing ones that you delete; you should keep that in mind while you're working.
Now, prepare for my Wall of Text Assault...
Ólan Stewart doesn't know who killed him, or why. All he knows is, after clawing his way out of his grave, he's now an undead blood-drinker*.
You need a hook first and foremost, although I'm sure you know that and are just working on the body of the query. With that assumption in mind, let's tackle the first paragraph...
The first two lines - spliced together - could make a good hook on their own, and they set up the rest of the query quite well, actually. I agree with the terminology; it doesn't actually matter in the query what terms you use, because the manuscript has its own words. You can use "vampire" without reprisal, and it's actually quicker and more easily understood. Don't worry about trying to sound too original; in fact, it'll make you look like you're trying too hard. Agents don't like to see "cute" in queries, they just want (rather, need) the facts.
A query also isn't the place to show off, which can also make you look like you're trying too hard. Just write smoothly and efficiently and you'll get attention. A flourish here and there in an incidental manner isn't bad, but trying to be different just for the sake of being different can be frowned upon.
Unwilling to stay in Edinburgh and risk harm to his family, he finds a new home in Eilean Donan castle. It seems more trouble than it's worth. Caring for Ciaran, a blind, traumatized blood-drinker, is enough; negotiating living arrangements with a human clan is almost too much.
Dropping too many names can create a word soup. It's not necessary to know the name of this castle, too, because it's only there for a short time in the query; strongly consider dropping it and making reference only to "a castle". Too many names will make the query bog down and can even make it confusing to follow.
The next two sentences - although cleanly written - leave some questions. Where did Ciaran come from? Why is Ólan caring from him? How did they meet? Why is he important to him? And why is Ciaran traumatized? Why does he need care? Is he an invalid? It also brings up the question of why he's trying to make arrangements with humans, when he tried to get away from some in the first place; he's not human, so why is he trying to live with them?
Ólan's hope for peace ravels when a clansman tries to murder Ciaran, and threatens to reveal the blood-drinkers to the other humans.
I read this sentence and think KKK. It would be clearer, though, to simply say that one of the humans finds out what he is and wants to expose them for some reason. But that reason needs to be stated; is he afraid of them? Angry? Does he want their kind eradicated? Is there a personal story regarding him? Don't name the man - he's not important enough - but his reasoning could use just a touch of exposition for clarity.
Ólan drives the man out and hunts him down, but another blood-drinker steals his kill.
Why does he drive him out? Why does he hunt him down? From the rest of the query, it sounds like he's reluctant to kill, so why does he try with this man? And isn't he reluctant to drink blood in the first place? His MO is vague right now, and this one piece sounds contradictory; the rest of the time, he seems harmless, but right here, he suddenly wants to kill a man. Why is that? Is he angry with him and trying to silence knowledge of his nature? Why the abrupt about-face?
The kill-thief leads Ólan to the City, a haven for their kind ruled by the ruthless Lord Conall.
Not to be too blunt, but "kill-thief" sounds contrived and juvenile. I might simply state that the vampire who kills the man leads them to..., or better yet, splice the two sentences together:
"... rescues him by killing the man, then leads them to..."
Or something similar. I always recommend, too, that you try not to use other writers' words in your query; you should let your own voice do the talking.
"A haven for their kind" also sounds a bit, eh, excessive; "a haven for vampires" works just as well. It has the added benefit of cutting one word.
There Ólan finds his father, held captive with dozens of other humans as food.
Good, good... although a flourish here could actually be appropriate. "...other humans, to be consumed at future feedings."
Or something to that effect. Again, your own voice could probably do better than mine. A bit of description can also help give a clear idea of the implications.
Ólan's greatest test comes when Conall orders him to feed from his father. Drinking human blood will start Ólan down a dark path which, if followed, will turn him as corrupt as Conall. Sparing his father and his soul will put the lives of everyone Ólan loves at risk, living and undead alike. Conall will kill as many innocents as he must to keep one dissenter in line.
There are some "buts" and "howevers" that could spice this up, but Crenshaw has good observations; it feels clipped and disjointed here, like you could definitely tell this in a much more flowing, dramatic manner. Let us feel the suspense! Something terrible is happening and a hard choice has to be made, but we're not feeling the tension here as currently written; it reads off more as facts, not suspense.
Right now, I'm thinking the last two lines can be reworded and restructured to provide a more intense leaving line, one that could give the query the needed punch it deserves.
This is a good start - a good framework - but definitely be careful with it if you don't have the manuscript completed; and be careful with the revision and added/deleted/modified scenes... they could change the content and character of parts of the query.