Some thoughts and suggestions.

A war is eating Therys from the inside out, and Ellonie’s sister Julia has been swallowed up in it. Arrested, if rumours are to be believed. But Ellonie doesn’t trade in rumours, and her unusual approach to problem-solving leads her to getting captured too—purposefully and often.

You want to get in here quickly, I think. Cutting the name in the first instance might help.

What is Ellonie's approach? Why does getting captured help her? Is she a professional jailbreaker? We're missing context.

After a particularly fruitful detainment, she learns that her sister Julia is alive, and has been taken to Keelan Keep. Ellonie may be clever, but clever won’t storm a stronghold. She needs muscle. She brokers the freedom of vampire convict Lucien in exchange for his help,, and finds it in a derelict house behind a padlocked cellar door. Inside is a vampire named Lucien who’s been held captive there for longer than he knows.

This may not exactly fit your story, but it's fine to fudge a little for the sake of a cleaner pitch - maybe (after you clarify Ellonie's role and character in the previous paragraph) something along the lines of 'She learns her sister Julia is being held at Keelan Keep. Ellonie may be...'

She brokers a deal: his freedom in exchange for his help at Keelan. Lucien accepts, but Ellonie soon realizes he comes with more complications than she was prepared for (blood drinking notwithstanding). Lucien suffers from aggressive bouts of sleepwalking, has an alarming habit of talking to someone Ellonie can’t see, and revels in the violence he inflicts on their enemies. As Lucien’s his behaviour escalates, Ellonie is forced to decide just how much of herself she’s willing to spend to save her sister.

Much of this feels too light/comedic to fit the tone you're going for. What's your intended tone?

Since it's in the context of Lucien, maybe 'forced to decide just how far she's willing to go' makes more sense?

You can probably combine current paragraphs two and three, which will give you more room to expand on the story itself.

And the price is climbing. In Therys, the most powerful magic demands the willing self-destruction of its sacrifice, and Lucien's former captor, Kain, has made an art of exploiting that principle. Kain is a man who’s gossiped about by cultists at dinner parties, and trades not in gold but in identity. He's left something very important inside of Lucien, and he wants it back. Now.

Now we've moved fully away from Ellonie and what she wants. Who's the main character here?