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.
When Celine dies in a plane crash, instead of going to heaven or hell, she becomes a character in a novel. This is a great concept, I love it. In the new world around her, I think we can assume the new world is around her - I'd strike the phrasing immortals gather energy from the environment to gain prolonged life. Devils suck the life out of mortals in exchange for power. And as the daughter of those two antagonistic races, Celine finds herself having to cover up her devil ancestry in her immortal school. She can live with all that, if she hadn’t read the novel. Oh, interesting. So she's actually living in a novel she's read before?
The novel is about how the protagonist Yinn becomes a powerful immortal. And Celine’s role as the rival of the protagonist is to contrast her failure with the protagonist’s success. But her failure in the novel means her death. This time, for good.
Her crush Erik, who also journeyed into this world, did he die too? tells her she should become a devil to change her fate. But her immortal mentor So is Yinn her mentor? The wording is a little confusing here would never approve of devils; he only slays them. When her doomsday comes closer, Celine must choose to embrace the devils or use what she knows to overturn the plot—and possibly the whole world in the process.
BOOK OF IMMORTALS: THE DISCIPLE is a complete 110,000-words fantasy novel with sequel potential.
This sounds fun. I like the idea that the protag already knows her fate. I don't have a lot of tips to offer you, other than perhaps explaining why / how Erik ended up in the novel as well. Overall this look good, my only trip-up would be the word count. Any debut author should try to aim for an under 100k word count. Fantasy gets a little leniency because of the world building, but 110k is still high. Try to trim down before you query widely.