Beta Demons or betademons are creatures that are endowed with extra sensory perception and are able to detect flaws in:

-Reality
-Continuity
-Geography
-History
-Spelling
-Grammer

They are able to see into alternate timelines of 'what may be' or 'what might have been'

They have the ability to shape and influence reality in a very unique way. They are able to shape what happens in the next book in the series.

To all intents and purposes, a betademon is a betareader. They get access to the next book before it is published. They are charged with reading the book and KEEPING IT A SECRET FROM ANYONE WHO IS NOT A BETADEMON!!!

The are required to read it carefully and look for issues of continuity, logic, pacing, awkward dialog, awkward scenes. Look for things that as a reader "Don't sit well." or "are not true to the character or story"

It is great if they can also find and report spelling and grammatical errors as this will make things much smoother for the Editor.

Editors and Beta Demons have different jobs.

Editors read for language structure, style and grammar. They make sure the writing is technically correct.
They look at the word, phrase, sentence and paragraph levels to ensure it flows properly.

A Beta Demon is more concerned with the overall story context, and while they do look for issues/errors/problems at the phrase, sentence and paragraph levels, they are more interested in "does this passage make sense in the context of the story?"

What a Beta Demon reads is not necessarily going to be the final "official version" of the book.

If a group of beta demons decide that a chapter or section of the book isn't working, doesn't make sense. That section may be edited/change/removed at the author's discretion.

This means that some events that happen in the beta version of the book, will never happen in the final book/timeline, if for example the betademons think it's stupid/lame or confusing.

This can bother some people. You have to be able to hold two versions of book reality in your mind at once. The final, published version which is 'reality' and the beta version which may have different events, more events, or fewer events than happen in official reality.

Things that happen in beta may not happen in reality, and vice versa. So be prepared for that. This is where most people flee.