In another universe, a young girl stumbles upon a plot in the kidnapping of children by a religious organization called the Magisterium, who will stop at nothing to ensure the continuation of their doctrine and hold over the world they live in.

Sounds like a terrific tale, Justin!

It’s a little long, I think, for a tag line, and there are a couple of things that come out straight away.

I don’t think we need “In another universe” — assuming that the book will be marketed as sci-fi or fantasy, that is pretty much a given.

I don’t think you need “called the Magisterium” — we’ll find that out when we start reading, right?

So, already it becomes tighter and clearer. Adding a couple of other changes I’d suggest:

A young girl stumbles upon a plot in the kidnapping of children by a religious organization, which will stop at nothing to ensure the continuation of their doctrine and hold over their world.

A couple of other thoughts: “stumble upon” tells us the character passively find out about this plot. I’d consider “a young girl discovers”, perhaps — it’s tighter in any case.

I also tightened “the world they live in” to “their world.”

Also, “stop at nothing” is a cliched phrase. Can you say the same thing with some different language?

Jeff